Investigation of abrasive wear resistance and wear mechanism of composite alloys

The abrasive wear resistance of composite alloys comprising hard tungsten carbide and soft CuNiMn matrix under different wear conditions has been investigated and compared with CrMo cast iron. It was found that Yz-composite alloy with hard cast angular tungsten carbide has greater wear resistance than CrMo cast iron under two-body wear conditions, but lower resistance than Cr-Mo cast iron under three-body wear conditions. It was found that under three-body wear conditions selective wear of the matrix and digging or fragmentation of tungsten carbide particles dominate in Yz-composite alloy, and microcutting and deformed ploughing is dominant under two-body wear conditions. The abrasive wear resistance of composite alloys under two-body wear condition is independent of bulk hardness, but is closely related to the microhardness of tungsten carbide.     

On evaluation of wear resistance of tooth enamel and dental materials

A survey of the literature shows that in many studies on the wear resistance of tooth enamel or dental materials a large scatter of experimental data has been obtained when wear tests were performed at a fixed load. Despite the steady loading, wear conditions vary during sliding, since tooth enamel as well as dental materials have inhomogeneous structure. This leads to changes in contact interactions between sliding surfaces, and as a result, we get changes in the friction and wear behaviour of tested materials. This is why at the same loading the wear can be different. In this study, more reliable approach to evaluation of the wear resistance of human enamel and dental materials is proposed. The procedure is based on the correlation between the volumetric wear and the friction energy dissipated during sliding. The model can be useful to compare the wear resistance of different dental materials tested in different ambient conditions.     

Sliding wear behavior of planar random zinc-alloy matrix composites

The friction and wear behavior of planar random zinc-alloy matrix composites reinforced by discontinuous carbon fibres under dry sliding and lubricated sliding conditions has been investigated using a block-on-ring apparatus. The effects of fibre volume fractions and loads on the sliding wear resistance of the zinc-alloy matrix composites were studied. Experiments were performed within a load range of 50–300 N at a constant sliding velocity of 0.8 m s⁻¹. The composites with different volume fractions of carbon fibres (0–30%) were used as the block specimens, and a medium-carbon steel used as the ring specimen. Increasing the carbon fibre volume fraction significantly decreased the coefficient of friction and wear rates of both the composites and the medium-carbon steel under dry sliding conditions. Under lubricated sliding conditions, however, increasing the carbon fibre volume fraction substantially increased the coefficient of friction, and slightly increased the wear of the medium-carbon steel, while reducing the wear of the composite.Under dry sliding conditions, an increasing load increased not only the wear rates of both the composite and the unreinforced zinc alloy, but also those of their corresponding steel rings. However, the rate of increase of wear with increasing load for both the composite and its corresponding steel ring was much smaller than for the unreinforced zinc alloy and its corresponding steel ring. The coefficient of friction under dry sliding conditions appeared to be constant as load increased within a load range of 50–150 N for both the composite and the unreinforced zinc alloy, but increased at the higher loads. Under any load the coefficient of friction of the composite was lower than half that of the unreinforced zinc alloy under dry sliding conditions.     

Comparison of wear properties of tool steels AISI D2 and O1 with the same hardness

Two commercial cold work tool steels, AISI D2 and O1, were heat treated in order to obtain the same hardness 700 HV (60 HRc) and were subsequently tested in three different modes of wear, namely in adhesion, three-body and two-body abrasion, by using pin-on-disk, dry sand/rubber wheel apparatus and pin abrasion on SiC, respectively. Even though AISI O1 and D2 steel are heat treated to the same hardness, they perform differently under the three modes of wear examined. The results show that the steel microstructures play the most important role in determining the wear properties. For relatively low sliding speeds AISI O1 steel performs up to 12 times better than AISI D2 steel in adhesive wear. For higher sliding speeds, however, this order is reversed due to oxidation taking place on the surface of the AISI D2 steel. The wear rate of both tool steels in three-body and two-body abrasion wear is proportional to the applied load. In three-body abrasive wear, AISI D2 exhibits a normalised wear rate about two times lower than the AISI O1 tool steel, and this is due to the presence of the plate-like hard carbides in its microstructure. Both tool steels perform 3–8 times better in three-body abrasive wear conditions than in two-body abrasive wear.     

人牙釉质的磨损机制研究

以钛合金为对摩材料,考察了人牙釉质在人工唾液介质中的滑动磨损进程,结合微观分析,研究了牙釉质的磨损机制。结果表明:在摩擦过程中,随着釉质表面发生脆性剥落,磨损从两体接触磨损转变为三体磨粒磨损,磨损加剧;随着接触时间增长,硬质磨屑尺寸变小,磨损率降低,进入稳定磨损阶段。牙釉质磨损为机械磨损过程,釉质表面基本无摩擦化学作用。     

Effect of age on the friction and wear behaviors of human teeth

Friction and wear behavior of human teeth at different ages against titanium alloy have been investigated using a reciprocating apparatus containing an artificial saliva solution. Human teeth at different ages of 8, 18, 35 and 55 years old were selected for tests. Both hardness and distribution of enamel rods on the occlusal surface, two of factors most important to tribological behavior of human teeth, are close to the age of teeth. It is found that not only the evolution of friction coefficient but also the wear behavior changes between teeth of different ages. Delamination and ploughing mechanisms are dominant for wear of human teeth, and more severe wear is observed for the primary and the permanent teeth at the old age accompanied by remarkable fluctuation in the friction coefficient. Compared with the primary teeth and the permanent teeth at the old age, the permanent teeth at the young and middle ages have better wear-resistance.     

食物粒度和硬度对人牙釉质三体磨粒磨损特性的影响

在往复滑动摩擦磨损试验台上对比考察了三体磨粒磨损过程中,食物颗粒的粒度和硬度对人牙釉质摩擦学特性的影响。结果表明:当食物颗粒硬度较高时,随着粒度减小,稳态摩擦因数略有降低,牙釉质磨损表面形貌逐渐由剥落为主转变为犁削效应;对于低硬度食物颗粒,随着粒度增加,稳态摩擦因数显著降低,牙釉质磨损表面形貌则由犁削效应转变为轻微擦伤。当食物颗粒粒度相同时,食物硬度对牙釉质的摩擦与磨损行为均有明显影响,食物颗粒硬度越高,稳态摩擦因数越高,磨损越严重。     

Fretting wear behavior of AZ91D and AM60B magnesium alloys

The fretting wear behavior of the AZ91D and AM60B magnesium alloys are investigated using a reciprocating fretting wear machine under dry conditions with different numbers of cycles, different normal loads, slip amplitudes and frequencies. The worn surfaces and wear debris were examined using scanning electron microscopy and optical microscopy in order to understand the predominant wear mechanisms of two magnesium alloys. The results indicate that the AZ91D alloy displays a lower friction coefficient and lower wear quantity than the AM60B alloy. The AZ91D shows a higher capability than AM60B in resisting crack nucleation and propagation. Both AZ91D and AM60B show similar friction and wear characteristics. The wear quantity increases with increasing normal load, but decreases with increasing frequency. The friction coefficient also decreases as the normal load is increased. Fretting frequency had little effect on the friction coefficient. In a long term, the fatigue wear and abrasive wear were the predominant wear mechanisms for AM60B and delamination wear, adhesive wear and abrasive wear for AZ91D.     

人牙釉质纳米尺度下的摩擦磨损行为研究

利用纳米划痕仪研究了人牙釉质在纳米尺度下的摩擦磨损行为,考察了法向载荷对牙釉质摩擦磨损性能的影响。结果表明:随着法向载荷增大,人牙釉质的摩擦因数和磨损深度呈现非线性增大;载荷较小时(20mN),摩擦因数随载荷增加而快速增大,划痕表面主要呈现轻微凹陷,损伤以弹塑性变形为主;当载荷较大时(20mN),摩擦因数随载荷增加而缓慢增大,划痕表面开始出现磨屑,磨损以脆性剥层为主。     

Study of <Emphasis Type="Italic">in vitro</Emphasis> wear of human tooth enamel

Wear behavior of human tooth enamel against titanium alloy has been investigated under the lubrication of artificial saliva, using a reciprocating sliding wear test machine with a normal load of 20 N, a reciprocating amplitude of 500 μm, and a frequency of 2 Hz. Tests lasting up to 10, 100, 1000, 2000, and 5000 cycles were conducted, respectively. The worn surface of enamel after different cycles was analyzed by means of various microscopic examinations. The results showed that, a delamination mechanism occurred on the surface of enamel at the early stage of wear. The wear rapidly increased in depth with the number of cycles. As the wear proceeded, the surface of enamel was gradually covered with a wear particle layer, and the wear rate decreased. Finally the wear of enamel stayed stable. Microscopic examinations indicated that the wear of enamel was controlled by the mechanical removal of materials without obvious changes in the compositions and crystal structures of the enamel. The enamel rods were worn away after the inter-rod enamel.     

Measurement, analysis and prediction of fretting wear damage in a representative aeroengine spline coupling

A methodology to predict fretting wear in complex couplings is described and validated against results obtained from a reduced scale aeroengine-type spline coupling subjected to complex cyclic load cases. The methodology uses three-dimensional finite element analysis, together with coefficient of friction data obtained from stroke controlled round-against-flat fretting tests, to determine spline tooth contact pressure and slip distributions; the latter as a function of number of loading cycles. A modified Archard equation is used to calculate wear depths from the contact pressure and slip distributions using wear coefficients obtained from the round-against-flat fretting tests. The slip distributions, and, concomitantly, the wear distributions, are found to depend strongly on the coefficient of friction, which, in turn, depends on the state of lubrication and number of loading cycles. For constant coefficient of friction, the slip distributions stabilise quickly with increasing numbers of loading cycles. The methodology predicts greater wear under lightly lubricated conditions than without added lubrication for the essentially load-controlled tests on the reduced scale aeroengine-type coupling. The wear depth trends are predicted correctly, both axially along the spline teeth and around the tooth flank, and the predicted wear depths bracket the measured values, dependent on the lubrication conditions considered; the latter attributable to the sensitivity of spline wear to the evolving coefficient of friction during testing. The methodology provides a basis for further development.     

A new tribometer for friction and wear studies of dental materials and hard tooth tissues

This paper presents a new tribometer developed for a study of the tribological behaviours of dental materials and hard tooth tissues. The device simulates oral kinematic conditions and the loading produced during masticatory process. The tribometer is similar to the existing devices regarding the kinematic features, i.e. it produces an adjustable oscillating movement. However, the device machine is equipped with a unique pneumatic system of loading controlled via computer by special software called TOOTHY. The programmable system allows easy adjusting of the loading parameters such as the magnitude of normal force and its amplitude or pattern of cyclic loading. In this way different combinations of loads can be applied thus making the investigation of different wear situations possible. The device has two full bridge strain gauges for the measurement of loading and friction forces, by which the coefficient of friction is determined.The use of the tribometer is illustrated by a comparative study of tribological behaviour of human enamel subjected to two- and three-body friction, and to two different loading patterns as well. The obtained results are discussed.     

Effect of saliva viscosity on tribological behaviour of tooth enamel

Employing a special device simulating conditions of the masticatory process, two series tests were conducted involving studies of the tribological behaviour of enamel subjected to two- and three-body tests at different saliva viscosities.The studies showed no significant influences of saliva viscosity on the mean of the friction coefficient. However, below 2.08 mPa s the coefficient was unstable, i.e. scatter was observed. Both higher wear rate and higher scatter were observed for viscosities below of 1.68 mPa s. Comparing the wear results of the three- and two-body tests, no significant differences were obtained for viscosities above 1.68 mPa s, although for lower viscosities the wear rate in the presence of abrasive particles was higher than that obtained with pure saliva.Two regions with different tribological behaviour of enamel were observed in relation to saliva viscosity; the region with stable and low values of the tribological parameters beyond some threshold value, and the other with higher mean values of tribological parameters and their scattering. The author suggests that different concentrations of proteins are possible at the same low saliva viscosity, thus there are qualitatively different interactions between enamel surface and saliva components resulting in different lubrication regimes.     

Friction and wear behavior of flame-sprayed PEEK coatings

Polyetheretherketone (PEEK) becomes of great interest to applications as bearing and slider materials. In this paper, PEEK coatings with three kinds of crystallinities were deposited using the flame spray process. Employing a uniform design experiment, the friction and wear behavior of the three PEEK coatings were systematically investigated under dry sliding conditions against a 100C6 counterbody on a ball-on-disc arrangement for several loads and sliding velocities. For the three coatings, the friction coefficient significantly followed the normal distribution. The average friction coefficients appeared to decrease while increasing the sliding velocity, but were insensitive to the applied load in the range of investigation. Among the three coatings, the higher the crystallinity of the coating, the lower its average friction coefficient was. The wear rate of the coating with the lowest crystallinity decreased with an increase in the load and a decrease in the sliding velocity. The wear rate of the coating with the intermediate crystallinity decreased with an increase in the load, but increased with an increase in the sliding velocity at lower loads, and then decreased with an increase in the velocity at higher loads. The wear rate of the coating with the highest crystallinity decreased with the increase of both the load and the sliding velocity. The wear mechanisms of the different coatings are explained in terms of plastic deformation, plogh marks and fatigue tearing.     

Hardness,friction and wear studies on hydrogen peroxide treated bovine teeth

A possible problem with peroxide based tooth whitening is the loss of tooth hardness and higher susceptibility to enamel surface wear. This study focussed on the effects of acidic and neutral hydrogen peroxide solutions (6 and 30% w/v) on hardness, friction and wear of bovine enamel. The experiments showed that treatments with neutral peroxide reduced wear and the loss of enamel hardness up to 2–3 times. In addition, further investigation on remineralisation with amorphous calcium phosphate showed an increase in hardness after treatment. Friction coefficients of teeth against steel varied between 0.25 and 0.7, and wear coefficients ranged between ≈10⁻⁶ and 10⁻⁷ mm³/N m. From this study, it is possible to explain the wear behaviour of HP treated enamel with changes in hardness.     

Gradation of oxidational wear of metals

This paper considers mild-oxidational wear of metals by studying their behavior under friction with different loads. Low carbon, steel and copper are chosen as the model materials. We show that tribo-oxidation and the structure of surface layers of materials, both formed in the process of plastic deformation during friction, provide the boundary conditions of mild and severe wear. Oxidational wear is predominant when structural changes are minimal. As the load increases, oxidational wear is at first accompanied by metallic wear and afterwards the oxidational wear accompanies the metallic wear. The structure of the metal surface layers changes gradually during these processes, so that the strengthening of the metal is high enough to withstand friction forces. When the magnitude of frictional forces becomes higher than the maximal strength of the plastically deformed metal, the transition to severe wear occurs.The composition of different types of oxides and the fineness of wear particles varies with the friction conditions. Under light load friction conditions, fine wear particles are formed. These particles contain oxides of high oxygen content. As the friction conditions become tougher, in particular when the load increases, large-sized wear particles are formed. These particles contain oxides of a higher metal content. Phase composition and fineness of wear particles are used for gradation of mild wear.Analyses of phase composition of oxides and estimation of the fineness of wear particles are suggested as a method of wear character diagnostics. The electron diffraction method of the study of wear particles is used for this analysis in order to evaluate and choose appropriate friction and wear conditions.     

Friction and wear behavior of electroless Ni-based CNT composite coatings

Ni-based carbon nanotube (CNT) composite coatings with different volume fraction (from 5 to 12 vol.%) of CNTs were deposited on medium carbon steel substrates by electroless plating. The friction and wear behavior of the composite coatings were investigated using a pin-on-disk wear tester under unlubricated condition. Friction and wear tests were conducted at a sliding speed of 0.0623 m s⁻¹ and at an applied load of 20 N. The experimental results indicated that the friction coefficient of the composite coatings decreased with increasing the volume fraction of CNTs due to self-lubrication and unique topological structure of CNTs. Within the range of volume fraction of CNTs from 0 to 11.2%, the wear rate of the composite coatings showed a steadily decreasing trend with increasing volume fraction of CNTs. Because of the conglomeration of CNTs in the matrix, however, the wear rate of the composite coatings increased with further increasing the volume fraction of CNTs.     

Effect of counterface roughness on abrasive wear of hydroxyapatite

The present study was undertaken to evaluate the effect of surface roughness of glass-infiltrated alumina on the abrasive wear of hydroxyapatite using a pin-on-disk tribometer. Hydroxyapatite was used as a model material to simulate tooth enamel. The wear tests were conducted in distilled water at room temperature using a constant sliding speed and three loads to model the normal occlusal contact conditions. The wear volume of polished hydroxyapatite pins increased by more than 20-fold as the average roughness of the alumina disks increased from 14 to 649 nm Ra. No measurable wear was detected on the alumina specimens. The wear surfaces were viewed in a scanning electron microscope (SEM) and were analyzed with X-ray dispersion spectroscopy to determine the extent of surface damage and the wear mechanisms. It is suggested that polished ceramic restorations will cause relatively low enamel wear, while increased roughness could severely abrade and damage the tooth enamel.     

Friction and wear of ceramics

The adhesion, friction, wear and lubricated behaviors of both oxide and non-oxide ceramics are reviewed. Ceramics are examined in contact with themselves, other harder materials and metals. Elastic, plastic and fracture behavior of ceramics in solid state contact is discussed. The contact load necessary to initiate fracture in ceramics is shown to be appreciably reduced with tangential motion. Both friction and wear of ceramics are anisotropic and relate to crystal structure as with metals. Grit size effects in two- and three-body abrasive wear are observed for ceramics. Both free energy of oxide formation and the d valence bond character of metals are related to the friction and wear characteristics for metals in contact with ceramics. Surface contaminants affect friction and adhesive wear. For example, carbon on silicon carbide and chlorine on aluminum oxide reduce friction while oxygen on metal surfaces in contact with ceramics increases friction. Lubrication increases the critical load necessary to initiate fracture of ceramics both in indentation and with sliding or rubbing.     

Fretting wear behavior of nanocrystalline surface layer of copper under dry condition

Unlubricated fretting tests were performed with a nanocrystalline surface layer of a 99.99 wt.% copper fabricated by means of surface mechanical attrition treatment (SMAT), in comparison with a coarse-grained (CG) copper. The measured friction and wear data show that the fretting wear resistance is markedly enhanced with the nanocrystalline surface layer relative to the CG counterpart. The friction coefficient and wear volume of the SMAT Cu are lower than that of the CG Cu. For both samples, the friction coefficients and wear volumes increase with an increasing applied load and fretting frequency. A rapid increase of the friction coefficient and wear volume under an applied load above a critical value (30 N for the SMAT Cu and 20 N for the CG Cu) is noticed, corresponding to the formation of a continuous oxide layer between two contact surfaces. Also two sharp increases of the friction coefficient and wear volume at fretting frequencies of 50 Hz and 175 Hz were observed for the SMAT and the CG Cu. The former is correlated with the formation of a continuous oxide layer, while the latter corresponds to wearing away of the oxide layer.