1 Introduction
The cross-integration of mechanical science and biomedicine has become a hot research topic. As an important component of living organisms, bone tissue plays an important role in supporting and weight-bearing [1], protecting internal organs [2], accomplishing movement [3], and participating in metabolism and hematopoiesis [4]. Due to the needs of patients with bone diseases and fracture treatment and plastic surgery, more and more attention has been paid to the cutting and treatment of bone tissue during surgery [5]. As a traditional precision machining method, grinding is increasingly being used in bone surgery to achieve the removal of bone tissue. Micro-grinding mainly refers to the use of micro-abrasives (the diameters of the abrasive shank and abrasive head are usually 3–6 and < 1 mm, respectively) [6] with a processing feature size of less than 0.05 mm to directly perform mechanical removal processing and form the desired shape [7]. It is an extension of the traditional machining process to the micro-scale. It plays an important role in the field of micromachining, with the advantages of high accuracy of the edges of machined parts and suitability for machining hard and brittle materials [8]. As a result, it is increasingly being used in orthopedic surgery to remove bone tissue. However, given that bone tissue is a typical anisotropic material, there are significant differences in the physical and mechanical properties in different directions [9]. Therefore, it is difficult to choose the best grinding speed that can reduce the bone temperature in actual surgery [10]. During clinical orthopedic surgical grinding, the grinding temperature of the bone material directly affects its biological activity and the degree of thermal damage to the surrounding soft tissues [11]. Mizutani et al. [12] proposed a cold air jet as cooling for bone grinding with miniature ball-end diamond wheels, which can prevent chip adhesion and stably suppress the temperature under a threshold temperature of 50 °C. However, when the temperature of the bone material in direct contact with the grinding tool reaches 47 °C and is maintained for more than 1 min, thermal necrosis will immediately occur due to high temperature [13]. Meanwhile, temperature higher than 43 °C will lead to thermal damage in the nerve tissue [14]. Thermal necrosis of bone material and surrounding soft tissues can prolong the patient’s postoperative recovery time [15]. Thermal injury caused by bone grinding temperature is a bottleneck problem in surgery. Currently, saline drip irrigation is commonly used in the grinding area to remove heat by convective heat exchange, thus achieving cooling of the hot area [16]. Kondo et al. [17] found through experiments that using saline flood cooling reduced bone grinding temperature and the spread of high-temperature areas. However, this method has a low cooling efficiency. It requires a large amount of coolant dripping in the grinding area, which tends to reduce the visibility of the surgical area [18]. Meanwhile, bone tissue composition contains a certain amount of polysaccharide proteins. The viscosity of polysaccharide proteins in grinding chips is enhanced at higher grinding temperatures [19], and under the extrusion of grinding tools and bone surfaces, grinding chips adhere to the surface of grinding tools, leading to clogging of grinding tools [20]. Therefore, there is an urgent need for a new grinding process to solve such problems.
In response to the problems of insufficient cooling capacity and poor visibility in clinical surgical bone grinding, researchers proposed the minimum quantity lubrication (MQL) grinding technique [21]. In the MQL grinding process, an extremely small amount of lubricating fluid and a gas with a certain pressure are mixed and atomized and then sprayed into the grinding area for cooling and lubrication [22]. MQL grinding can effectively reduce friction and energy consumption of the workpiece surface [23], reduce wear and tear, greatly improve the working environment [24], and reduce pollution to the natural environment [25], and it is an efficient and green processing technology. However, the cooling performance of MQL high-pressure airflow is limited and cannot meet the heat transfer requirements in the high-temperature environment of the grinding zone [26]. The machining quality of the workpiece and the life of the grinding tool are still far from those of physiological saline drip grinding, and this technology still needs further development [27]. Nanoparticle jet mist cooling (NJMC) is an upgrade and optimization of MQL [28]. According to the enhanced heat transfer theory, the heat transfer capacity of solids is much better than that of liquids and gases [29]. Therefore, nanoscale solid particles are selected to make nanofluid. The nanoscale solid particles, lubricating fluid, and high-pressure gas are mixed and atomized, and sprayed into the grinding zone in the form of a jet [30]. Zhang et al. [31] fabricated nanoparticles with different concentrations of molybdenum disulfide (MoS2), carbon nanotubes (CNTs), and their mixtures (MoS2-CNTs). Nickel-based alloy, a difficult-to-machine workpiece material, was used to experimentally study the grinding performance in the MoS2-CNT MQL. The results showed that the surface quality and machining accuracy of the workpiece were significantly improved due to the lubricating property and high thermal conductivity of the nanoparticles. Moreover, the grinding performance of the nanoparticles was studied by comparing the addition of MoS2 or CNT to the MQL grinding fluid and nanoparticles without concentration. Yang et al. [32] added hydroxyapatite, silica (SiO2), Fe2O3, and carbon nanotubes to physiological saline and investigated the effect of different nanoparticles on bone grinding temperature under nanoparticle jet minimum quantity cooling conditions. The results showed that the different nanoparticles had different thermophysical properties, which led to different bone surface temperatures. The nanoparticles could degrade naturally in the human body several months to a year after the end of the procedure. Their medicinal components were absorbed by the body, playing a supplementary therapeutic role. SiO2 nanoparticles are the most typical and widely used nano-drug carriers with good biocompatibility and mechanical properties in the biomedical field. As a structurally simple nanomaterial, SiO2 nanoparticles could introduce a variety of functional groups through surface modification, and they are widely used in biomedical fields such as drug carriers and drug release due to their good biocompatibility, high specific surface area, and chemical stability [33] (Fig.1). The NJMC method added medical nanoscale solid particles to the lubricating fluid, which improved the proportion of heat transfer to the outside environment to a certain extent. However, the nanoparticle content was very small, and its enhanced heat transfer effect was poor and needed further improvement [34].
Given the low effective flow rate and tendency to cause clogging of grinding tools during micro-grinding, ultrasonic vibration (UV)-assisted grinding has been proposed [35]. UV-assisted grinding uses high-frequency impact to change the kinematic and thermodynamic properties between the grinding tool and the workpiece material to obtain superior grinding performance. UV-assisted grinding is an effective way for machining of difficult-to-cut materials [36], such as Ni-based superalloy [37], TC4 titanium alloy [38], and brittle bone materials [9]. The process is popular among researchers and clinicians due to the advantages of miniaturized equipment and high stability. Cao et al. [39] studied the vibration coupling effects and machining behavior in UV-assisted grinding of Inconel 718 Ni-based superalloy. Meanwhile, the grinding force and machined surface quality of UV-assisted grinding and conventional creep-feed grinding were compared and analyzed. The results showed that UV-assisted grinding had better grinding performance than conventional creep-feed grinding [40]. Alam et al. [41] carried out UV planar cutting of cortical bone and studied the effect of different vibration parameters on cutting forces. Gupta and Pandey [42] studied the drilling forces and torques of bone at different rotational speeds, feed rates, drill diameters, and UV amplitudes. The results showed that the rotational UV bone drilling method could minimize the forces and torques during bone drilling. Babbar et al. [43] used a hybrid cumulative equivalent minute (CEM43 °C) and Arrhenius model to compare and analyze the causes of thermal damage to human tissues during conventional grinding and UV-assisted grinding of bone materials. The study showed that UV-assisted grinding could effectively reduce the grinding force and grinding temperature, inhibit the clogging of grinding tools, and improve the quality and efficiency of workpiece grinding. UV grinding reduced the grinding temperature during bone grinding and could effectively prevent osteonecrosis and nerve damage.
UV further improved the grinding performance of MQL-assisted grinding. Li et al. [44] investigated the grinding performance of different vibration frequencies and amplitudes from the viewpoint of grinding surface quality and tool life using an experimental approach. The results showed that the best surface quality could be obtained using a high frequency (11.4 kHz) and a low feed rate. UV could extend the tool life by two times compared with conventional grinding. In addition, MQL significantly improved tool life in UV-assisted grinding. However, there was a gap between the selected parameters and the requirements to produce UV in a strict sense. Therefore, the excellent machining effect of UV may not be fully exploited. Jia et al. [45] found that the combination of multi-angle two-dimensional (2D) UV and NMQL techniques significantly reduced the adhesion and material peeling on zirconia ceramics. Rabiei et al. [46] reported that UV and NMQL compounding could reduce the grinding temperature from 254 to 132 °C without any thermal damage or burning compared with dry grinding. Yan et al. [47] experimentally investigated the effects of dry, UV, and MQL compounding processes on the turning properties of Ti‒6Al‒4V alloy materials. They found that the tool contact length was reduced, and the chip shape was optimized under the condition of a continuous MQL with UV system. Gao et al. [48] evaluated the surface properties of GH4169 nickel-based alloy using 2D UV-assisted grinding and NMQL coupling. The results showed that the surface roughness was reduced by 19.5% and 39.9% for both couplings compared with UV and NMQL, respectively. The kinematic model of 2D UV was also established to simulate the relative motion trajectories of the abrasive grains and the workpiece under different UV angles.
However, the current research on UV and NMQL compounding process is mainly focused on metal, ceramic, and other materials. Research related to the micro-grinding of biological bone with complex structure and anisotropic physical and mechanical properties is lacking. Therefore, to investigate the effectiveness of ultrasonic vibration-assisted nanoparticle jet mist cooling (U-NJMC) micro-grinding in biological bone, a grinding experimental study was conducted on fresh bovine tibial dense bone, which has the most similar mechanical properties to human bone. The influence of different grinding conditions (dry grinding, drip, UV, MQL, NJMC, and U-NJMC) on the grinding performance of biological bone were designed and researched. Grinding force, grinding force ratio, friction coefficient, and grinding temperature were used as characterization parameters to study the effect of UV and NJMC on the thermal damage of micro-grinding force. The active control strategy of thermal damage of micro-grinding force was investigated to provide technical support for clinical surgery.
2 Experimental
2.1 Experimental setup
The experimental setup was a U-NJMC micro-grinding bone surgery experimental platform, which mainly included an axial UV system and an NJMC system, as shown in Fig.2. The main technical parameters consisted mainly of a spindle power of 2.5 kW, a maximum spindle speed (n) of 24000 r/min, a workbench with dimensions of 600 mm × 1200 mm × 800 mm, an axial vibration amplitude (A) of 2–8 μm, and a frequency (f) of 18–22 kHz. In the NJMC system, the nanoparticles and saline could be uniformly distributed by UV bar. After the two were mixed in the mixing chamber and prepared into a low-concentration nanofluid, the nanofluid was ejected from the electrostatic atomization nozzle by the MQL pump. Then, the nanofluid droplets were charged and atomized by the adjustable high-voltage direct current power supply to form a cluster of charged micro-droplets, which were transported to the workpiece surface in a controlled and orderly manner driven by the electric field force and mainly played the role of lubrication and cooling in the grinding motion. The 3D grinding force dynamometer (YDM-III99, Dalian University of Technology, China) was used to measure the tangential, normal, and axial grinding force in real time, as shown in Fig.3(a). The dynamometer is connected to the charge amplifier using the matching high-transmittance leads and to the A/D data acquisition card installed on the PC. DynoWare software was used to measure and record the grinding force. In the acquisition of grinding force data, the data collection frequency should be as high as possible, so that it can collect detailed grinding force information as much as possible. The sampling frequency of 1 kHz can obtain the best grinding force information based on equipment performance and our previous work in the laboratory [3,31]. In addition, low-pass filtering of DynoWare was used to denoise the obtained grinding force signal. Taking the center of the grinding force sampling point in the effective grinding area as the benchmark, 100 data points were selected as the basic data of the grinding force analysis. The average value was calculated to obtain the corresponding force average value. The data selection method described above ensured the representativeness of the grinding force signal. The bone grinding surface temperature was measured by the perforated buried full artificial thermocouple method. The blind hole was drilled at the central axis on the other surface of the workpiece. Under micro-grinding condition, the shallow grinding depth (ap = 0.015 mm) easily led to the penetration of the workpiece surface when the blind hole was drilled. Thus, the distance between the bottom of blind hole and the top surface is 0.3 mm. To achieve accurate grinding temperature, the micro-grinding was executed several times at the same grinding conditions until the top surface’s location moved to the bottom of the blind hole [49], as shown in Fig.3(b).
2.2 Workpiece materials and abrasives
Because of the hard and brittle characteristics of biological bone, a diamond grinding tool with abrasive grains of 200# mesh was selected as the micro-grinding tool. The average grain size was 75 μm. The electroplated diamond micro-grinding rod with stable processing quality and good overall performance was selected. The diameter of the grinding head and tool handle was 1 and 3 mm, respectively. Given that the composition of bovine bone is basically similar to that of human bone and the physiological and mechanical properties are similar to those of human bone tissue, the dense bone in the middle of the bovine tibia was used as the experimental material in this study. Fresh bovine tibia from 2–3-year-old adult cattle purchased from a slaughterhouse was soaked in physiological saline, and the soft tissue attached to the external surface of the tibia was removed [50]. A section of the tibia with relatively uniform diameter was taken and prepared into bone samples with size of 40 mm × 10 mm × 5 mm. The samples were polished with grit sandpaper to remove any damage caused during the sample preparation process so that the surface to be processed was flat and smooth and then treated with saline. Then, the samples were used for the experiments immediately or placed in a −20 °C freezer to maintain their thermophysical properties for subsequent experiments [51]. However, all specimens were kept for a maximum of one week to avoid changes in tissue properties over time, making the experimental results non-comparable. Also, in order to ensure that the bone samples could recover their biological properties during the experiments, the bone samples to be processed in the freezer needed to be removed and placed at room temperature for 1 h to re-warm before the next experiment. Bone material is a typical anisotropic material (Fig.4) with significant differences in physical and mechanical properties of different directions. The radial, axial, and tangential structures differ significantly, so the mechanical properties and grinding characteristics must also differ when micro-grinding is performed on different faces in different directions. For this reason, three directions (axial, radial, and tangential) of biological bones were selected to study the micro-grinding characteristics of bone tissue in different directions.
2.3 Nanoparticle materials
Saline was used as the base fluid, and SiO2 nanoparticles were used as the nanoscale solid additive with an average particle size of 20 nm. The suspension stability of nanofluids is the best when the volume fraction is 2 vol.% [11]. Nanoparticles made the nanofluid’s thermal conductivity much higher than that of liquid saline due to the excellent heat transfer properties. Moreover, the rolling effect and weak shear surface between molecular layers of nanoparticles made the nanofluid exhibit excellent tribological properties [48]. Polyethylene glycol 400 (PEG400) with a volume fraction of 1/10 was added as a surface dispersant and assisted with UV to improve the stability of the suspension [7]. SiO2 nanoparticles were mixed with physiological saline to prepare a low-concentration nanofluid. The specific implementation method is as follows: First, 1.2 g of SiO2 nanoparticles was added into 1000 mL of normal saline. Then, 2 mL of PEG400 dispersant was added. Finally, after mechanical stirring, the sample was vibrated in an ultrasonic vibrator for 15 min to make a nanofluid with good dispersion performance, and the test was carried out immediately. The nanofluid was sprayed to the bone grinding area, which guaranteed the safety to the human body and served as an auxiliary cooling function. Before starting the test, the ultrasonic generator should first be turned on. The UV would interfere with the heat generated by the workpiece grinding process at this time, so it was necessary to wait for the UV to reach a stable state before conducting the grinding experiment, which would not happen with conventional micro-grinding.
2.4 Experimental conditions
Six grinding processes, namely, dry grinding, drip irrigation, UV, MQL, NJMC, and U-NJMC, were performed to verify the performance of U-NJMC micro-grinding. Based on pre-trial experience and previous studies [52,53], drip grinding was made of physiological saline with a liquid flow rate of 50 mL/h. The nanofluid was prepared by a two-step method, and further details of this preparation method were reported in Ref. [52]. The amplitude and frequency of UV were set to 7.5 μm and 20 kHz, respectively. Further details on this testing method can also be found in Ref. [53]. The details of experimental lubrication parameters under different lubrication conditions for grinding are shown in Tab.1.
Tab.1 Lubrication parameters under different lubrication conditions |
| No. | Lubrication conditions | Lubrication parameters |
|---|---|---|
| 1 | Dry | ‒ |
| 2 | Drip | Liquid flow rate Q = 50 mL/h |
| 3 | UV | Axial vibration amplitude A = 7.5 μm, frequency f = 20 kHz |
| 4 | MQL | Q = 10 mL/h, air pressure P = 0.5 MPa, nozzle angle α = 45°, injection distance D = 15 mm |
| 5 | NJMC | Q = 10 mL/h, P = 0.5 MPa, α = 45°, D = 15 mm, nanofluid: 1.2 g SiO2 + 2 mL PEG400 + 1000 mL saline |
| 6 | U-NJMC | A = 7.5 μm, f = 20 kHz, Q = 10 mL/h, P = 0.5 MPa, α = 45°, D = 15 mm, nanofluid: 1.2 g SiO2 + 2 mL PEG400 + 1000 mL saline |
In order to maintain experimental consistency, the processing parameters were kept constant except for the different lubrication conditions, as shown in Tab.2.
Tab.2 Experimental scheme of micro-grinding process |
| No. | Grinding process parameters | Numerical value |
|---|---|---|
| 1 | Grinding tools | Micro-grinding |
| 2 | Spindle speed n | 21000 r/min |
| 3 | Feeding speed vw | 120 mm/min |
| 4 | Grinding depth ap | 0.015 mm |
3 Results and discussion
3.1 Effect of bone tissue orientation on grinding force
The grinding force is an important physical quantity to evaluate the material removal characteristics in the grinding process [54]. It directly affects the surface quality and damages the biological bone material, which seriously affects the healing of bone tissue after surgery. At the same time, it can also provide a basis for the wear of abrasives and facilitate the timely replacement of abrasives. To estimate the grinding force of bone tissue during micro-grinding in different directions, diamond abrasive was used, and the grinding parameters were selected as shown in Tab.2. Micro-grinding was performed on the side, section, and surface. To ensure the accuracy of the measurement results, each group of parameters was measured five times, and the statistical values were taken [55]. The results of the axial, tangential, and normal grinding forces are shown in Fig.5, where Fa, Ft, and Fn are the axial grinding force, tangential grinding force, and normal grinding force, respectively.
The results showed that Fn and Ft were the largest with 7.19 and 3.23 N, respectively, and Fa did not differ significantly when grinding the cross-section. Fn and Ft were 5.59 and 1.87 N, respectively, which decreased by 22.2% and 42.1%, respectively, with respect to the cross-section. Fn and Ft were the smallest when grinding the surface and decreased by 28.5% and 49.2% with respect to the cross-section, respectively. This was mainly because bone tissue is an anisotropic material. The dense bone can usually be considered as a fibrous toughened composite material, which was the external support tissue of the bone and consisted of osteon, interstitial lamella, circumferential lamellae, cement line, and Haversian system arranged in an axial direction. Large amounts of bone osteon and the inter-, inner, and outer circumferential lamellae in the dense bone increase the slip resistance of the bone material during plastic deformation, exponentially increasing the energy absorption capacity and improving the fracture toughness of the bone material in the cross-section grinding process. The grinding resistance of the abrasive grains and the friction between the abrasive grains and the specimen were larger. This means that more energy needed to be accumulated to grind and propagate through the bone unit under this direction compared with the other two grinding directions. Thus, the grinding force during cross-sectional grinding was the largest, as shown in Fig.6(a). In addition, the bone-bonding line was the weakest part of the dense bone due to the presence of the bonding line boundary around the bone unit. Under the action of the grinding force, the bone material was extremely peeled away from the bone matrix; it produced deflection and distortion along the bond line and extended into the interior of the bone material along the bond line [56]. Some microcracks turned into macroscopic cracks, which had a destructive effect on the bone unit system and affected the biological activity of the bone material and its regenerative capacity. Due to the co-existence of cracks expanding along the long axis direction of the bone unit and cracks expanding along the grinding direction, the grinding chips produced in this mode were fragmentary. When grinding the lateral side, the grinding direction was consistent with the bone unit direction, and the grinding resistance was much smaller when grinding the bone unit. The grinding direction was perpendicular to the annular bone plate direction, which was cross-sectional grinding relative to the inner and outer annular bone plate. As shown in Fig.6(b), the grinding resistance was larger. The grinding direction was consistent with the bone unit direction and the annular bone plate direction when grinding the surface. The cutting resistance was much smaller when grinding on the bone unit and the annular bone plate. Hence, the grinding force during surface grinding was minimal, as shown in Fig.6(c). Furthermore, as the angle between the bone unit distribution and the bone long-axis direction varied between 5° and 15°, there was always an angle between the bone unit long-axis direction and the grinding direction [56]. During surface grinding, the direction of crack propagation in the grinding direction was highly susceptible to propagate upward/downward along the bond line boundary toward the interior of the material. Therefore, when the crack propagation encounters the bone unit, the crack propagation changes along the direction of the bone unit inclination rather than directly through the bone unit.
3.2 Effect of different working conditions on grinding force
The experimental research was carried out on the side of the specimen using diamond micro-abrasives to investigate the effect of different grinding conditions on the grinding force. Typical measured signals of grinding force under six different working conditions were obtained, as shown in Fig.7. Fn and Ft of drip, MQL, UV, NJMC, and U-NJMC grinding were reduced compared with dry grinding, indicating that the use of grinding fluid or UV during the grinding process could reduce the grinding force. The grinding force obtained by U-NJMC micro-grinding was the smallest and showed certain advantages. To accurately represent the magnitude of the grinding force under various lubrication conditions, five sets of experiments were repeated under each lubrication condition, and 100 consecutive data points were intercepted in each group of grinding force stabilization phase and processed for mathematical and statistical analysis. Fig.8 shows the statistical values of the grinding force under the six lubrication conditions. Fn and Ft obtained by dry grinding were 5.59 and 1.87 N, respectively. Fn and Ft of NJMC condition were 3.41 and 0.90 N, respectively. Compared with conventional dry grinding, Fn and Ft in NJMC grinding were reduced by 39.0% and 51.9%, respectively. Fn and Ft in UV grinding were 2.62 and 0.67 N, respectively. Compared with conventional dry grinding, Fn and Ft of UV were reduced by 53.1% and 64.2%. Fn and Ft of U-NJMC were 1.39 and 0.32 N, respectively. Compared with dry grinding, U-NJMC could decrease Fn and Ft by 75.1% and 82.9%, respectively. The grinding force was the highest during the dry grinding process due to the absence of any lubricant. Drip took away the grinding chips with a large amount of grinding fluid to reduce friction, which made the grinding force lower than that in dry grinding. MQL could improve the visibility of the grinding area. Still, only a small amount of saline grinding fluid carried away the grinding chips to reduce friction, which made the trend of reducing the grinding force less obvious. The lubricating property of SiO2 nanoparticles in NJMC reduced the friction between the grinding tool and the workpiece and decreased the grinding force, which was significantly less effective than NJMC. The lubrication effect of MQL was obviously inferior to that of NJMC as it only relied on a small layer of saline lubricating fluid in MQL grinding.
The friction coefficient μ is the ratio of Ft and Fn, which can intuitively reflect the lubrication effect between the sliding interfaces in the grinding zone. The coefficient of friction μ was expressed as follows:
Fig.9 shows μ under six different lubrication conditions. Dry grinding had the maximum friction coefficient (). For the rest of the grinding conditions, drip, MQL, and NJMC based on water-based lubricants showed a slightly lower friction coefficient (, , ), whereas the friction coefficient under UV grinding was still reduced compared with water-based lubricants. U-NJMC has the smallest μ () among all grinding conditions. Compared with dry, drip, MQL, UV, and NJMC grinding, the U-NJMC condition friction coefficient was reduced by 31.3%, 17.0%, 19.0%, 9.8%, and 12.5%, respectively.
In addition to the grinding force and friction coefficient, the specific grinding energy es is the most common parameter to evaluate the grinding performance. It refers to the total system energy consumed per unit time for grinding to remove a unit volume of workpiece material. It also reflects the lubrication effect of the grinding/workpiece interface. The smaller es is, the better the lubrication effect will be. es was expressed as follows [57,58]:
where vs is the grinding tool linear speed, and is the micro-grinding workpiece width (Fig.10) and could be expressed as follows:
where r is the radius of the abrasive.
Fig.11 shows es under six different grinding conditions. Dry grinding consumes the most energy, and es was as high as J/mm3, which increased Ft and generated more energy consumption. U-NJMC grinding consumed the least energy, and es was 104 J/mm3. Compared with dry, drip, MQL, UV, and NJMC-assisted micro-grinding conditions, es in U-NJMC grinding decreased by 83.0%, 72.7%, 77.8%, 52.3%, and 64.7%, respectively. The reason for the larger es corresponding to drip cooling lubrication compared with the U-NJMC process was that the high fluid dynamic pressure in the grinding zone prevented the water-based grinding fluid from entering the grinding zone, resulting in insufficient lubrication at the grinding tool/workpiece interface, which made the grinding force larger, thereby consuming more energy. In the case of NJMC, under the action and the penetration of the capillary network in the grinding area, the nanoparticles entered the grinding area well, which made the grinding tool/workpiece interface fully lubricated and reduced the grinding force, resulting in a reduction of the energy consumed.
3.3 Grinding temperature
In the process of bone micro-grinding, human tissue might suffer not only from mechanical damage caused by direct contact with the grinding tool or vibration during grinding but also from thermal damage caused by grinding. Most of the energy consumed by the mutual collision between the abrasive grains and the bone material was converted into heat energy and collected in the grinding zone during bone grinding. Excessive grinding temperature could easily lead to thermal damage of bone cells. Therefore, the grinding temperature under different working conditions was investigated. Fig.12 shows the grinding temperature under the six cooling conditions. The grinding temperature of dry grinding was the highest, with a peak of about 43.6 °C. The peak grinding temperature of UV-assisted micro-grinding was 4.2 °C lower than that of dry grinding. During UV-assisted micro-grinding process, the coolant was easier to pump into the grinding zone with high-frequency, alternating positive and negative hydraulic shock waves, which accelerated the renewal of coolant in the grinding zone and made the grinding temperature in the grinding zone lower. At the same flow rate, the temperature peaks of the two MQL cooling methods without UV were 5.7–14.5 °C lower than those of the dry grinding condition due to the cooling and lubricating characteristics of the grinding fluid. However, compared with drip cooling, the effect was less obvious, but it could also transfer and take away some heat to achieve the cooling effect. The peak temperature of MQL grinding was 37.9 °C, whereas that of NJMC was 29.1 °C. Under the NJMC and MQL grinding process, the concentration of grinding fluid was the same, and the main reason for the grinding temperature difference was that the thermal conductivity of solid nanoparticles was greater than that of liquid nanoparticles. Therefore, the peak grinding temperature of NJMC was lower than that of MQL, which was mainly due to the high specific surface and heat capacity of nanoparticles that enhanced convective heat transfer and tribological properties in the grinding zone. U-NJMC had the lowest grinding temperature with a peak of about 26.2 °C. Compared with UV and NJMC alone, the grinding temperature of U-NJMC was reduced by 33.5% and 10.0%, respectively. This indicated that the coolant in the grinding zone accelerated the renewal under the combined action of UV and NJMC grinding fluid. Consequently, this technology greatly enhanced the convective heat transfer capacity of the cooling medium and significantly reduced the grinding temperature in the grinding zone of the machining process. Finally, the experimental findings for different grinding conditions with the corresponding grinding force (Fn and Ft), coefficient of friction (μ), specific grinding energy (es), and grinding temperature (T) are summarized in Tab.3.
Tab.3 Summary of micro-grinding biological bone at different lubrication conditions with the corresponding experimental results |
| Working conditions | Fn/N | Ft/N | μ | es/(J∙mm−3) | T/°C |
|---|---|---|---|---|---|
| Dry | 5.59 | 1.87 | 0.335 | 2.47 × 104 | 43.6 |
| Drip | 4.23 | 1.17 | 0.277 | 1.54 × 104 | 29.8 |
| UV | 2.62 | 0.20 | 0.284 | 0.88 × 104 | 39.4 |
| MQL | 5.04 | 1.43 | 0.255 | 1.89 × 104 | 37.9 |
| NJMC | 3.41 | 0.23 | 0.263 | 1.19 × 104 | 29.1 |
| U-NJMC | 1.39 | 0.32 | 0.230 | 0.42 × 104 | 26.2 |
Combined with the above experimental results, the grinding heat could not be dissipated in time due to the low thermal conductivity and thermal diffusivity of the bovine tibia material. It resulted in a gradual increase in grinding temperature, which in turn caused thermal necrosis of bone cells and loss of regeneration ability. The heat exchange capacity of the cooling medium in the confined space of the bovine tibia/micro-grinding tool was insufficient because of the lack of cooling and lubricating medium involved in the material removal process due to dry grinding. This resulted in a sharp increase in the micro-grinding temperature of the bovine tibia, which led to local water loss in the bone material and increased friction [59]. Therefore, biological bone dry grinding had poor processing performance and was prone to affect the biological activity of surrounding tissues, which triggered thermal damage to bone cells, nerves, and blood vessels around the grinding area, affecting the postoperative recovery of patients. Although a large amount of grinding fluid was involved in the grinding process in physiological saline drip micro-grinding, the low supply pressure and low initial kinetic energy made it difficult to break through the air barrier around the high-speed rotating grinding tool, which made its effective utilization rate into the grinding area low and only provided effective cooling and lubrication to the areas near the sides of the grinding tool. Moreover, diamond abrasive grains are often used as surgical grinding tools. Diamond has hydrophobic characteristics, which is even more unfavorable for saline injection into the grinding zone.
The grinding force of UV-assisted micro-grinding was lower than that of dry grinding mainly due to the cyclic reciprocation of UV frequency during UV-assisted grinding to achieve high-frequency intermittent grinding, whose schematic of UV-assisted micro-grinding is shown in Fig.13 [60]. Given that the number of separations was the same as the frequency, the periodicity of UV caused the tool and the workpiece to separate periodically, and the coefficient of friction was greatly reduced at the moment of the appearance of the gap in the UV process, thus reducing the average friction between the tool and the material.
In normal grinding, the thickness of the undeformed chip of a single abrasive grain (ag) is calculated as follows [61]:
where is the contact arc length between the grinding rod and the workpiece material, C is the effective number of abrasive grains per unit area, and θ is the average cone half angle of the abrasive grains.
In normal grinding, of a single abrasive grain is related to and the abrasive diameter [62]:
The UV changed the cutting path length of a single abrasive grain during UV-assisted micro-grinding. Therefore, the contact arc length between the grinding rod and the workpiece material in UV-assisted micro-grinding (l2), which can be expressed as follows [62]:
where φ is the initial phase of UV, and φ is usually taken as 0. If is used instead of in Eq. (5), the undeformed chip thickness of UV-assisted micro-grinding is the thickness of the abrasive grain. Given that , UV reduced the undeformed chip thickness of a single grinding grain during grinding, which in turn made the grinding force of UV-assisted micro-grinding smaller than the normal grinding force. Furthermore, based on the vacuolar flow, stress wave, and impact kinetics theories, the abrasive tool of UV also produced cavitation and pumping effects at the abrasive tool/bone grinding interface. Microstructural damage, such as hydrogen bond breaking, occurred especially in high-protein and collagen tissues during ultrasonic high-frequency impact [63]. This process was often accompanied by heat generation, which accelerated the local liquefaction of biological tissue. The cavitation effect generated by the ultrasonic impact, which occurred mainly in the cutting process of relatively soft tissues such as fat, assisted the ultrasonic cutting effect [64].
The cavitation effect also helped to protect the surrounding tissues and reduced the surrounding tissue additional damage [65]. In addition, the cavitation effect of UV increased the self-cleaning of abrasives and enhanced the heat transfer capability of coolants. The pumping effect made it easier for medical coolant to enter the grinding zone and promoted the circulation of coolant in the grinding zone. In summary, UV softened the workpiece and changed the grinding properties of the workpiece, resulting in a reduction in the undeformed thickness during grinding. By contrast, the introduction of UV reduced the friction coefficient between the workpiece and the abrasive grains, decreasing the friction force and thus making the overall grinding force lower [66].
NJMC lubrication conditions had the participation of SiO2 nanoparticles. The incorporation of nanoparticles provided excellent wear reduction and anti-friction effect on NJMC grinding, which was mainly related to its laminar molecular structure. As shown in Fig.14, there were three types of hydroxyl groups on the surface of SiO2 nanoparticles: undisturbed isolated hydroxyl groups, conjoined hydroxyl groups forming hydrogen bonds with each other, and twin hydroxyl groups with two hydroxyl groups attached to one silicon atom. The Si−O activity in the molecular structure of SiO2 nanoparticle was related to the position it occupies. At the center of the structure, the Si−O bond was polar and had a high binding capacity. The Si−O bonds that were at the surface of the particles were highly active and capable of force binding interactions with other molecules. Fig.15 shows the lubrication mechanism of SiO2 nanoparticles [60]. In the micro-grinding process, the uniformly dispersed spherical SiO2 nanoparticles can play the role of “bearing-like rolling” between the friction pairs in a certain load range. In other words, the friction coefficient was reduced by changing the sliding friction into rolling friction, as shown in Fig.15(a). Under the action of compressive and tangential stresses in the grinding process, SiO2 nanoparticles with high surface activity were strongly chemisorbed on the workpiece grinding surface through hydroxyl groups, forming a solid SiO2 lubricant film, as shown in Fig.15(b). Nanoparticles had a larger specific surface area due to their small particle size, with an average particle size of 20 nm, which was tens or even thousands of micron-sized particles. Such a high specific surface resulted in an increased number of atoms on the surface. At the same time, the surface energy increased rapidly. Due to the increased number of surface atoms, the unsaturation of atomic coordination led to large numbers of unsaturated bonds and suspension bonds, making nanoparticles have high activity and large extremely unstable surface energy. As shown in Fig.15(c), the nanoparticles formed a lubricating layer on the workpiece surface by diffusion and penetration. When the load increased, the nanoparticles reacted chemically under the action of frictional heat and formed a chemically reactive film on the frictional surface, thus enhancing the Brownian motion, prompting the fluid to break away from the bone surface faster and intensifying the heat transfer by increasing the perturbation of the boundary layer [67]. This made the grinding temperature decrease. However, the number of particles increased as the volume fraction increased. The closer the particles were to each other, the greater the interaction, which led to the enhanced random motion of nanoparticles. The heat exchange rate and energy transfer within the nanofluid were enhanced, which strengthened the heat transfer characteristics of the nanofluid. At the same time, the nanoparticles would gather and fill the pits formed on the surface of the friction subsets, which improved the lubrication performance between the friction subsets, thus enhancing the lubrication effect in the grinding zone, as shown in Fig.15(d). Therefore, U-NJMC could effectively solve the high grinding force and grinding temperature of biological bone micro-grinding and provided a new way for clinical surgical cranial micro-grinding.
4 Conclusions
Grinding experiments were performed under six different working conditions: dry, drip, UV, MQL, NJMC, and U-NJMC. The grinding force, coefficient of friction, specific grinding energy, and grinding temperature were measured and compared. The following conclusions were obtained:
(1) The grinding force differs in different directions under the process of biological bone micro-grinding. The section grinding force was the largest, followed by the side grinding force. The surface grinding force was the smallest, and Fn and Ft decreased by 28.5% and 49.2% with respect to the cross-section, respectively.
(2) Compared with dry grinding, the grinding temperature of UV-assisted micro-grinding was reduced by 9.6%, and that of NJMC micro-grinding was reduced by 33.3%. Fn and Ft of UV-assisted micro-grinding were 2.62 and 0.67 N, which were reduced by 53.1% and 64.2%, respectively. Compared with dry grinding, Fn and Ft of NJMC micro-grinding were 3.41 and 0.90 N, which were reduced by 39.0% and 51.9%, respectively. Compared with dry, drip irrigation, MQL, UV, and NJMC conditions, the friction coefficient of U-NJMC condition was reduced by 31.3%, 17.0%, 19.0%, 9.8%, and 12.5%, respectively.
(3) Experiments of biological bone micro-grinding under six grinding conditions (dry, drip, UV, MQL, NJMC, and U-NJMC) were carried out. The results showed that U-NJMC obtained an Ft of 0.32 N, Fn of 1.39 N, friction coefficient of 0.230, and grinding specific energy of J/mm3, which provides a new technical reference for the application of micro-grinding technology in orthopedic surgery.