The friction of diamond sliding on diamond

Measurements are reported of the friction of diamond styli polished to a spherical tip sliding over a flat polished diamond surface. Particular attention was paid to maintaining standard conditions during the experiments, particularly the crystallographic orientations of the styli, the flat surface, and the directions of sliding, as well as the conditions of polish. The coefficient of friction was determined for sliding on both (001) and (011) faces, in different sliding directions, and for a range of loads and tip radii. The value of the friction and its variation with the direction of sliding depend quite strongly on the magnitude of the load and the radius of the stylus. However, the present results show that styli of different radii give quite similar friction when sliding under the same mean contact pressure. Hence, apparent discrepancies between previous measurements of the friction may be related to different regimes of pressure in the different experiments. When the stylus slides in the direction of easy abrasion of the flat the coefficient of friction passes through a pronounced minimum value as the contact pressure is increased. This behaviour suggests that at least two mechanisms contribute to the friction. A discussion based on the unusual topography of polished diamond surfaces, shows that the forces and energy losses associated with the friction may arise via at least three different mechanisms. The main features of the present results may be accounted for by two of these mechanisms in which surface asperities either ride over each other or push each other aside. (The third mechansim involving only fracture of the asperities appears to make no significant contribution.)     

超声振动对单晶硅锯切比能的影响

超声振动能很好地改善硬脆性材料的加工性能,为了探索超声振动锯切比能对单晶硅的影响,本文采用薄金刚石锯片,在有无超声振动的条件下对单晶硅进行锯切实验.实验结果表明:超声振动使锯切材料过程中的比能大幅度降低;2种锯切方式下锯切比能都随着单颗磨粒最大锯切厚度的增大而降低,但普通锯切方式下锯切比能呈幂指数递减趋势,而在超声振动的作用下比能变化趋势转变为良好的线性递减;并且单晶硅材料的去除方式由普通锯切中塑性去除为主导转变为脆性断裂去除,其破碎方式属于微破碎,趋于粉末状破碎,由此在不会对工件表面产生严重损伤的同时使材料去除所消耗的能量得到了有效降低.同时,超声振动使得锯片上的磨粒对单晶硅表面的高速冲击作用,使单晶硅产生大量微裂纹,对单晶硅的微小剥离起到很大作用.因此,超声振动在单晶硅材料的加工中有着很大的发展前景.     

Interaction potential and friction of hydrogenated diamond surfaces at the atomic scale: first-principle calculation

Interaction potential plays a vital role on the friction. Potential energy of two contacts directly determines on the friction force. However, many mathematic models proposed, always abandon the instantaneous status during sliding to devote to a weighted average over lateral force, which might miss some information about friction behavior. In this work, the relation among potential energies, separation distances of two contracts, and positions in the sliding direction are studied to evaluate the instantaneous friction characteristics. Two hydrogenated diamond surfaces are used as the examined model. The results show that a watershed between positive and negative friction forces locates at the position with the minimum adsorption energy. A rapid decrease in potential energy occurs near the 2.5 nN external force, where the friction coefficient approaches zero at each position in the sliding direction. Therefore, a novel method may be developed to greatly reduce the friction coefficient between two surfaces by adjusting the contract pressure.     

多约束条件下金刚石圆锯片结构优化研究

基于序列响应面模型和混合优化算法相结合的方式,提出一种在复杂载荷条件下降低金刚石圆锯片锯切噪声的优化方法。建立了多变量、多约束条件下金刚石圆锯片结构优化的数学模型,经计算得到了满足刚度要求的优化圆锯片。通过数学统计分析,获得了尺寸参数对锯片的声学、变形以及应力等性能影响规律。根据结果制备了金刚石圆锯片,锯切实验表明优化锯片能够有效的降低噪声,表明这种方法可为锯片或此类结构的声学优化研究提供新的方向和参考依据。     

On the formation of the diamond grain configuration during high temperature creep and fatigue

A study has been made of the influence of test variables on the formation of the diamond grain configuration during high temperature creep and fatigue deformation of a wide variety of metals. The proposed mechanisms for the formation of this interesting grain morphology are reviewed. It is concluded that the diamond grain configuration arises from a balance between grain-boundary sliding, grain-boundary mobility, intragranular deformation and defect imbalance across the grain boundaries and that it tends to be stabilized by intergranular cavitation. While the phenomenon occurs during high temperature fatigue in a variety of metals irrespective of their crystal structure, during creep it has been observed only in to h c p metals. It is surmised that the occurrence of the diamond array of grain boundaries during creep deformation in h c p metals is aided by the limited number of slip systems which leads to high defect imbalances in adjacent grains and consequently high driving forces for grain-boundary migration. On the basis of quantitative metallography involving measurements of the number of edges per grain section, the number of grains meeting at vertices, angular distribution histograms and grain-boundary lengths in different angular orientations with respect to the stress axis in "annealed" and "diamond" microstructures, it is concluded that the shape of the "diamond" grain is essentially the same as that of the "annealed" grain but in a distorted form.     

Development of diamond coated tool and its performance in machining Al-11% Si alloy

An attempt has been made to deposit CVD diamond coating on conventional carbide tool using hot filament CVD process. ISO grade K10 turning inserts with SPGN 120308 geometry were used to deposit diamond coating. This diamond coating well covering the rake surface, cutting edges and flank surfaces could be successfully deposited. The coatings were characterized by SEM, XRD and Raman spectroscopy for coating quality, morphology etc. Performance of diamond coated tool relative to that of uncoated carbide tool was evaluated in turning Al-11% Si alloy under dry environment. The diamond coated tool outperformed the uncoated carbide tool which severely suffered from sizeable built-up edge formation leading not only to escalation of cutting forces but also poorer surface finish. In contrast, the diamond coated tool, owing to chemical inertness of diamond coating towards the work material, did not show any trace of edge built-up even in dry environment and could maintain low level of cutting forces and remarkably improved surface finish. It has been further revealed that success of the diamond coated tool depends primarily on adhesion of the diamond coating with the carbide substrate and this is strongly influenced by the pre-treatment of the carbide substrate surface before coating.     

Study of friction and wear mechanisms at high sliding speed

The aim of this work is to propose an analysis of mechanisms inducing surface interaction by friction during high sliding speed. Specific devices including a ballistic setup were used to reproduce extreme sliding conditions combining high speed and high pressure. The titanium alloy/tantalum tribo-pair is chosen to investigate the frictional and material transfer mechanisms. The tangential force measurement is used to follow the evolution of the friction coefficient at a macroscopic scale. The evolution of the sliding surface was analyzed by confocal 3D microscope to evaluate material transfer and real contact surface area. Numerical modeling of micro-contact at the asperities scale is presented to illustrate the scenarii involved during friction. The energy needed to shear a junction is estimated and analyzed for several types of interaction. Different behaviors have been taken into account in order to investigate the global forces generated by the contact including strong and weak contacts. The analysis of energy is available to predict the global friction force in a large range of velocities. Correlations between experimental measurements and numerical predictions are used to validate the proposed approach. The results can be interpreted as following: (1) at lower velocity the main mechanism dominating the interaction between asperities becomes ploughing with large volume of plastic deformation (2) at higher velocity the main mechanism is shear localization requiring less energy and force for shearing the junctions.     

Investigation of the atomic-scale friction and energy dissipation in diamond using molecular dynamics

We have used molecular dynamics simulations to examine friction when two diamond (111) surfaces are placed in sliding contact. The essence of atomic-scale friction was shown to be the mechanical excitation (in the form of vibrational and rotational energy) of the interface lattice layers upon sliding. This excitation was propagated to the rest of the lattice, and eventually dissipated as heat. In general, this excitation increases with increasing applied load; therefore, the atomic-scale friction also increases with load. Flexible hydrocarbon species, chemically bound to the diamond surface, can lead to a significant reduction of mechanical excitation upon sliding at high loads, leading to lower friction. In addition to clarifying the effects of chemically-bound hydrocarbon groups on atomic-scale friction at diamond interfaces, these simulations might also yield insight into more complicated systems, e.g. Langmuir-Blodgett films, and aid in the design of low-friction coatings.     

Materials Sliding Wear Model Based on Energy Dissipation

A mathematical model is developed to correlate the volumetric wear of materials with the dissipation energy in sliding contacts. In the analysis, the wear of contacting materials originating from the energy loss due to friction process in the contact is studied. Two mechanisms responsible for energy loss at contact are considered. The first is the amount of energy spent to import plastic deformation and the second is the elastic energy of the particulate. The energy loss due to elastic and plastic deformation is calculated. The statistical loss of energy is calculated for two rough surfaces by the assumption that there is negligible change in the statistical parameters of the surface during wear. The model can be useful to predict the service lifetime of components and eventually structures. The results showed that the amount of dissipated energy and the volumetric loss increased with increasing normal load. Also, changing the normal load changed the rate of energy dissipation per unit sliding distance.     

CHARACTERISTICS OF MICROCUTTING FORCE VARIATION IN ULTRAPRECISION DIAMOND TURNING

An investigation of the characteristics of microcutting forces in diamond turning of crystalline materials is presented. The characteristics of the cutting forces were extracted and analyzed using statistical and spectrum analysis methods. A series of cutting experiments were done on a copper alloy and copper single crystals with different crystallographic orientations. Experimental results indicate that there exists a dominant frequency component and a periodicity of fluctuation of the cutting forces per workpiece revolution in the diamond turning of a single crystal material. The periodicity is closely related to the crystallographic orientation of the material being cut. As the depth of cut increases, the influence of crystallographic orientation of the single-crystal materials on microcutting forces is found to be more pronounced. Moreover, the cutting force ratio between the mean thrust force and the mean cutting force is found to vary with the depth of cut, and a large ratio was observed at a small depth of cut. These findings help to explain quantitatively the periodic fluctuations of microcutting forces (and hence the materials-induced vibration) in ultraprecision diamond turning, which are not encountered in conventional machining.     

超声振动辅助单晶硅划片的锯切力研究

采用金刚石超薄锯片对单晶硅划片的工艺会在划片时产生较大锯切力,从而导致较大的崩边损伤。而旋转超声辅助加工由于超声振动的作用可以减小加工时所产的切削力,同时获得较好的加工精度,越来越广泛地应用于硬脆材料的加工中。为了验证超声辅助对单晶硅划片中锯切力的作用,在实验中将超声振动添加到锯片上,使其产生径向的振动来完成对单晶硅的划片。并通过对比有超声振动辅助与无超声振动辅助的单晶硅划片的锯切力,对超声振动辅助划片中锯切力的特点进行分析。实验结果表明,超声辅助划片所产生的锯切力比无超声辅助划片所产生的锯切力小,说明超声振动的添加可以降低锯切力。同时在超声划片中产生的崩边要小于非超声加工条件下的崩边情况,说明超声振动降低锯切力可抑制硅片的崩边。     

A Grain Orientation-Independent Single-Step Saw Damage Gettering/Wet texturing Process for Efficient Silicon Solar Cells

Improving electrical and optical properties is important in manufacturing high-efficiency solar cells. Previous studies focused on individual gettering and texturing methods to improve solar cell material quality and reduce reflection loss, respectively. This study presents a novel method called saw damage gettering with texturing that effectively combines both methods for multicrystalline silicon (mc-Si) wafers manufactured using the diamond wire sawing (DWS) method. Although mc-Si is not the Si material currently used in photovoltaic products, the applicability of this method using the mc-Si wafers as it contains all grain orientations is demonstrated. It utilizes saw damage sites on the wafer surfaces for gettering metal impurities during annealing. Additionally, it can crystallize amorphous silicon on wafer surfaces generated during the sawing process to allow conventional acid-based wet texturing. This texturing method and annealing for 10 min allow for the removal of metal impurities and effectively forms a textured DWS Si wafer. The results show that the open-circuit voltage (ΔV_oc = +29 mV), short-circuit current density (ΔJ_sc = +2.5 mA cm⁻²), and efficiency (Δη = +2.1%) improved in the p-type passivated emitter and rear cells (p-PERC) manufactured using this novel method, as compared to those in the reference solar cells.     

Some observations on the wear of single point diamond tools used for machining glass

The wear pattern on a single point diamond tool used for machining glass is studied. Possible wear mechanisms are proposed on the basis of additional sliding wear tests and observations by scanning electron and optical microscopy. The wear process is believed to take place in two stages, one involving a polishing mechanism and the other consisting of crack propagation which occurs after the accumulation of a certain amount of damage. The importance of the crystallographic orientation of the diamond single crystal, particularly for crack propagation along cleavage planes is pointed out. It is concluded that the likelihood of rapid deterioration of the diamond tool may be decreased by proper crystallographic orientation.     

Meso-scale dislocations and friction of shape-complementary soft interfaces

The interface between two surfaces patterned with complementary shapes such as arrays of ridge–channel structures or pillars accommodates relative misorientation and lattice mismatch by spontaneous production of dislocation arrays. Here, we show that the relative sliding of such an interface is accomplished by dislocation glide on the interfacial plane. An exception is the singular case where the lattices are perfectly matched across the sample dimension, in which case sliding is accompanied by motion of edge-nucleated defects. These are meso-scale analogues of molecular sliding friction mechanisms between crystalline interfaces. The dislocations, in addition to the long-range elastic energy associated with their Burgers vectors, also cause significant out-of-plane dilation, which props open the interface locally. For this reason, the sliding friction is strongly pressure dependent; it also depends on the relative orientation of the patterns. Sliding friction can be strongly enhanced compared with a control, showing that shape-complementary interfaces can be engineered for strongly enhanced pressure- and orientation-dependent frictional properties in soft solids.     

Molecular dynamics simulation model for the quantitative assessment of tool wear during single point diamond turning of cubic silicon carbide

Silicon carbide (SiC) is a material of great technological interest for engineering applications concerning hostile environments where silicon-based components cannot work (beyond 623 K). Single point diamond turning (SPDT) has remained a superior and viable method to harness process efficiency and freeform shapes on this harder material. However, it is extremely difficult to machine this ceramic consistently in the ductile regime due to sudden and rapid tool wear. It thus becomes non trivial to develop an accurate understanding of tool wear mechanism during SPDT of SiC in order to identify measures to suppress wear to minimize operational cost.In this paper, molecular dynamics (MD) simulation has been deployed with a realistic analytical bond order potential (ABOP) formalism based potential energy function to understand tool wear mechanism during single point diamond turning of SiC. The most significant result was obtained using the radial distribution function which suggests graphitization of diamond tool during the machining process. This phenomenon occurs due to the abrasive processes between these two ultra hard materials. The abrasive action results in locally high temperature which compounds with the massive cutting forces leading to sp³-sp² order-disorder transition of diamond tool. This represents the root cause of tool wear during SPDT operation of cubic SiC. Further testing led to the development of a novel method for quantitative assessment of the progression of diamond tool wear from MD simulations.     

Recent Advances in Disordered and Nanostructured Carbon Coatings for Superl Ubricity and Wearless Sliding

Increasingly more demanding and very stringent operating conditions envisioned for future mechanical and tribological systems will certainly require new materials and coatings that are superhard and at the same time self-lubricating.For example, dry machining is a much desired practice in manufacturing sector, but it is currently very difficult to realize mainly because of high friction and severe wear losses. However, recent advances in surface engineering and coating technologies may enable design and production of novel coatings architectures that can combine superhardness with self-lubricating properties in both the disordered or nanostructured forms. Recently developed nearly frictionless carbon films, ultrananocrystalline diamond and carbide derived carbon films can dramatically lower friction and at the same time reduce wear under very harsh sliding conditions. These coatings can be formulated in such a way that they can substantially increase the load-bearing capacity of sliding surfaces and hence improve their resistance to scuffing. It is also possible to design nano-composite coatings that can form self-replenishing and-lubricating tribofilms on their sliding surfaces and thus help increase the overall lubricity of these surfaces. In this paper, an overview of recent advances in disordered and nanostructured carbon films will be presented. Specific examples will be given to demonstrate the superior performance and durability of such novel coatings under a very wide range of tribological conditions. The major emphasis is placed on super low friction carbon films. The fundamental tribological mechanisms that control their exceptional friction and wear behaviors are also discussed.     

Direct observation of frictional seizure of mild steel sliding on aluminum by X-ray imaging Part II Mechanism

Observation of frictional contacts has always been a problem for long as the contact is normally hidden. In this work, we have used an X-ray microscope for in-situ observation of frictional seizure, wear and interfacial features during the testing of mild steel specimens sliding against Al 6061 disk. This technique enables the observation of interfacial features of the hidden contact. Seizure tests were conducted at different sliding speeds of 2, 4 and 5 m/s. The images obtained during the tests indicated that the wear process was a combination of random transfer events and cyclic process of a close contact followed by a partial separation of the sliding surfaces. Wear was concentrated over a certain specific area during the initial part of the test but later the contact developed into a conformal contact following a lumpy transfer of material. The mechanisms of seizure and wear were affected by the sliding speed. At a sliding speed of 4 and 5 m/s, the transfer and bonding of material was not directly caused by nascent surface contact but due to contact of rolled and compacted wear debris with the nascent surfaces. Whereas at lower sliding speed (2 m/s) the transfer and bonding of deposits occurred due to direct contact of nascent sliding surfaces.     

Superior wear resistance of diamond and DLC coatings

As the hardest known material, diamond and its coatings continue to generate significant attention for stringent applications involving extreme tribological conditions. Likewise, diamond-like carbon (DLC, especially the tetragonal amorphous carbon, ta-C) coatings have also maintained a high level interest for numerous industrial applications where efficiency, performance, and reliability are of great importance. The strong covalent bonding or sp³-hybridizaiton in diamond and ta-C coatings assures high mechanical hardness, stiffness, chemical and thermal stability that make them well-suited for harsh tribological conditions involving high-speeds, loads, and temperatures. In particular, unique chemical and mechanical nature of diamond and ta-C surfaces plays an important role in their unusual friction and wear behaviors. As with all other tribomaterials, both diamond and ta-C coatings strongly interact with the chemical species in their surroundings during sliding and hence produce a chemically passive top surface layer which ultimately determines the extent of friction and wear. Thick micro-crystalline diamond films are most preferred for tooling applications, while thinner nano/ultranano-crysalline diamond films are well-suited for mechanical devices ranging from nano- (such as NEMS) to micro- (MEMS and AFM tips) as well as macro-scale devices including mechanical pump seals. The ta-C coatings have lately become indispensable for a variety of automotive applications and are used in very large volumes in tappets, piston pins, rings, and a variety of gears and bearings, especially in the Asian market. This paper is intended to provide a comprehensive overview of the recent developments in tribology of super-hard diamond and DLC (ta-C) films with a special emphasis on their friction and wear mechanisms that are key to their extraordinary tribological performance under harsh tribological conditions. Based on the results of recent studies, the paper will also attempt to highlight what lies ahead for these films in tribology and other demanding industrial applications.     

Preferential etching by flowing oxygen on the {100} surfaces of HPHT single-crystal diamond

Application of diamond is determined by its oxidation behaviour in some measure. Oxidation process of single-crystal diamond prepared under high pressure and high temperature (HPHT) has been studied by the thermal analysis, scanning electron microscope (SEM) and Raman spectrometer. The result of a simultaneous thermal analysis indicates that single-crystal diamond is oxidized at ~ 818°C at a heating rate of 5°C/min in the flowing oxygen. Based on the data of the thermal analysis at different heating rates, the activation energy is calculated by the Kissinger method. A weight loss rate increases with the rising heat treatment temperature from 600 to 800°C. After the oxidation at 800°C, etch pits emerge on the {100} surfaces of single-crystal diamond, while the {111} surfaces are smooth. Shapes of the etch pits on the {100} surfaces are inverted pyramidal hollows, with edges direction parallel to the <110> direction.