Stephen L. Rice

Characteristics of metallic subsurface zones in sliding and impact wear

Abstract Various metallic pairs were tested under conditions of unlubricated solid contact. Experiments were conducted for repetitive impulsive and continuous sliding contact. Wide ranges of materials and conditions (nominal contact stress and relative transverse sliding velocity) and a variety of loading modes (pure normal impact at various frequencies, compound impact at various sliding velocities, and pure sliding under various stress levels) were explored. Particular attention was focused on the establishment of subsurface material zones developed in the tests, in situ . These zones exhibit dependences on velocity, stress, material, test duration and loading mode. The experimental findings, based on several analysis techniques, serve to characterize subsurface zone composition and morphology. Both surface and subsurface features were examined by optical and electron microscopy and analyzed by energy-dispersive X-ray techniques to allow interpretations concerning the role of external parameters, material transport and debris formation, as well as insight into operative mechanisms which act on specific materials under prescribed conditions to cause wear.

The role of microstructure in the impact wear of two aluminum alloys

Abstract The impact wear resistance of two aluminum alloys was investigated using flat-ended aluminum specimens impacted upon a stainless steel counterface. The counterface itself was held stationary in some tests (pure normal impact) and moved transverse to the normal impact direction in other tests (compound impact). The alloys investigated were aluminum-copper: 2011-T3 which was formulated for free-machining applications and 2124 which possessed very high fracture toughness. Thus, one alloy favors crack nucleation and growth, while the other suppresses these. A variety of tests were conducted with both alloys in compound impact loading. The peak impulsive stress was found to influence wear rates significantly; the relative sliding velocity is also an important parameter. Surface and subsurface microscopy were used to define operative wear mechanisms. With the 2011-T3 alloy, the characteristic subsurface features support the delamination theory of wear. With the 2124 alloys, subsurface features differ significantly. These features are discussed in the light of microstructural variations in the alloys.

Formation of Subsurface Zones in Impact Wear

Recent studies have established that subsurface material structures are altered in processes giving rise to wear. This is the case both for sliding contact and repetitive compound impact loading. It is not surprising that the thermal and mechanical processing due to solid contact and relative motion lead to localized material property change. Yet subsurface material characteristics (such as degree of plastic deformation, void and crack content, layer formation, grain refinement, etc.) may be of considerable significance in terms of fundamental wear processes. Accordingly, a series of tests has been undertaken to determine the onset of characteristic subsurface zone formation arising in repetitive compound impact wear. The repetitive load cycling is convenient for such investigations (incubation studies) since a contact event is discretized a priori. The test series and subsequent analyses seek primarily to determine the dynamics of formation of subsurface material zones, and also to characterize these zon...

Reciprocating impact wear testing apparatus

Abstract A reciprocating impact wear testing apparatus has been developed for studies of wear occurring between solid materials undergoing repetitive impulsive loading. Such loading may be purely normal or may include simultaneous transverse sliding. The apparatus is designed to allow measurement of the impact load pulse and to provide for the maintenance of repeatable force pulses for the duration of a prolonged series of impacts (millions of cycles). When utilized with a flat-nosed impacting specimen, the time-invariant impulse waveform results in repeatable surface and subsurface stress cycling of materials undergoing wear. This paper describes the testing apparatus and procedures and includes results from a preliminary series of tests on two polymeric materials.

The role of microstructure in the wear of selected steels

Abstract This investigation is a continuation of impact wear studies which focus on the nature of subsurface microstructure. Both AISI 1045 and 2.25Cr-1Mo steels were selected for their capacity to form various phase morphologies at given compositional states. Heat treatment was then performed to produce the desired two-phase (duplex) structure in both materials. The mating counterface to each test material was a 17-4 PH stainless steel in the martensitic condition. Compound impact wear tests were performed at relative transverse sliding velocities of 1 and 10 m s −1 with peak nominal contact stress maintained at 69 MPa for various numbers of repetitive load cycles. The formation and characterization of subsurface zones were studied by scanning electron microscopy and energy-dispersive X-ray analysis. Wear debris was inspected by powder X-ray diffraction. The impact wear resistance of AISI 1045 and 2.25Cr-1Mo steels is dependent on transverse velocity. Variations in velocity lead to “trade offs” between specimen and counterface 17-4 PH stainless steel wear which is evidenced in weight loss data and correlates with microstructural observations (subsurface zone formation) for each two-phase system. Wear debris analysis confirms the presence of mechanochemical material interaction between specimen and counterface with increasing transformation and oxidation at the higher transverse sliding velocity.

The role of specimen stiffness in sliding and impact wear

Abstract The results of pin-on-disc sliding tests and of impact wear tests are presented. Titanium alloy specimens were used for the sliding tests and high strength steel specimens were used for the impact tests: in both types of test stainless steel was the counterface material. The test duration, the nominal contact stress and the effective “stiffness” of the pins were varied; the effective stiffness was varied by changing the unsupported length of the pins while all other experimental conditions were maintained invariant. Experimental data on the wear track depth and roughness, obtained by profilometry, and on the specimen and counterface surfaces, which were examined by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) techniques, are presented: the data include morphological features and material transport observations. Subsurface sections of specimens were analyzed by SEM, EDX techniques and transmission electron microscopy. Characteristic subsurface zones are identified and described with respect to both morphology and composition in the near-surface microstructure. The data clearly indicate that the stiffness is an important factor in experimental work in sliding as well as in impact wear.

Comparative in vitro Sliding-wear Study of Conventional, Microfilled, and Light-cured Composite Restoratives

The sliding-wear behavior of a number of commercially available microfilled and light-cured composite restoratives has been investigated and compared with the wear characteristics of conventional composites. The surface profiles of the composite restoratives, both pre- and post-test, have also been examined in order to study material attrition processes. The results indicate that: (1) microfilled composites are significantly more resistant to sliding wear than are restoratives containing conventionally-sized filler particles; (2) light-cured, microfilled composites exhibit a lower rate of sliding wear than do self-curing, microfilled restoratives; and (3) the wear process leads to an increase in surface roughness for all materials tested, but microfilled materials display lower surface roughness values, both before and after sliding-wear tests, than do restoratives containing conventional-sized filler particles.

Specimen material reversal in pin-on-disc tribotesting

Abstract Results from pin-on-disc wear tests in which specimen and counterface materials were interchanged are presented in this paper. In one set of tests, pins of material A were slid against discs of material B; then, in another set of tests, pins of material B were slid against discs of material A. There are many factors to consider in such experiments, including the fact that the pin experiences a greater sliding distance and presumaly higher near-surface temperatures. However, test results from such specimen material reversals are useful in studying the roles of thermal, mechanical and chemical processes in sliding wear and also in assessing the validity of tribotesting against a “standard” counterface material. The results reported include data on volume change (obtained from weight change measurements), on wear track profiles, on subsurface zone morphology and composition and on debris composition. As expected, these data indicate that significantly different results will be obtained for a given pin-on-disc test for pin material A against disc material B compared with pin B against disc A.

Influence of Contact Stress, Sliding Velocity, and Surface Roughness on the Sliding Wear of a Composite Restorative

The influence of several experimental parameters on the sliding-wear behavior of a composite restorative has been examined. The results demonstrate that: 1) Changes in surface finish and sliding velocity have little effect on the moderate wear-rate observed at nominal levels of stress, and 2) increased contact stress can profoundly alter wear mechanisms and produce marked surface failure at levels well within the range associated with human mastication.

Material transport phenomena in the impact wear of titanium alloys

Abstract Results are presented from compound impact wear studies performed with titanium alloys of different β phase content and morphology. The “material pair” consisted of a 17-4 PH stainless steel counterface and a flatended titanium alloy specimen. Each material pair was exposed to variations in relative transverse sliding velocity and number of repetitive load cycles. Testing was conducted primarily at a single level of nominal peak normal impulsive stress. Both optical and scanning electron microscopy were used to monitor changes in worn surface and subsurface regions. Energy-dispersive X-ray studies of the initial and worn surfaces comprising the material pair clearly indicated the nature of the material transport between the opposing surfaces. Wear debris were studied by optical microscopy and by powder X-ray techniques. Utilizing the reciprocating impact wear testing apparatus, it was determined that material transport appears to be a controlling factor. The type of transport ( i.e. material passing from specimen to counterface or vice versa) was found to vary under differing test conditions. Such findings may contribute to the understanding of wear for systems other than those characterized by repetitive impulsive contact. Material removal is minimal at particular levels of relative transverse sliding velocity, and these levels are not necessarily affected by the magnitude of the nominal level of stress. It appears that the nature and quantity of the constituents (α, β) in the titanium alloys are critical in establishing wear behavior for the material pairs investigated.