R.I. Trezona

Transitions between two-body and three-body abrasive wear: influence of test conditions in the microscale abrasive wear test

Abstract The microscale abrasive wear test (also known as the ball-cratering wear test) is generally considered to be a three-body wear test. Nevertheless, different test conditions can produce either two-body (grooving) or three-body (rolling) wear mechanisms. The wear mechanisms and wear rates were investigated over a range of loads (0.1 to 5.0 N), slurry concentrations (0.000031 to 0.24 volume fraction abrasive) and abrasive materials (SiC, Al 2 O 3 and diamond). It was found that for each abrasive, a transition from grooving to rolling wear could be identified with a critical ratio of load to slurry concentration. The wear rate varied with concentration, with a maximum at intermediate slurry concentrations. The classification of abrasive wear into two-body and three-body mechanisms is discussed with reference to the problems noted by Gates [J.D. Gates, Two-body and three-body abrasion: a critical discussion, Wear 215 (1998) 139–146].

Three-body abrasive wear testing of soft materials

Abstract This paper investigates some of the problems associated with determining the `three-body (rolling particle) abrasion resistance of soft materials such as polymers and soft metals. The ball-cratering micro-scale abrasion apparatus, initially developed for testing hard coatings, has been used to study the behaviour of soft materials: polymethylmethacrylate (PMMA), commercially pure aluminium, tool steel and, as a reference hard material, glass-bonded alumina. The importance of the initial entrainment of abrasive particles and its effect on the outcome of the test is discussed with reference to the hardness and roughness of the spherical counter-body. The morphology and evolution of the wear scar have been carefully studied. Problems associated with the measurement of the scar and subsequent calculation of the wear rate are identified, and solutions to these are suggested. The effects of normal load on particle entrainment and wear rate for soft materials are also described. Recommendations are made for the optimum test conditions for micro-scale abrasion testing of soft materials.

The effects of ball surface condition in the micro-scale abrasive wear test

The ball‐crater micro‐scale abrasive wear test can be applied to coated samples and small bulk samples. The surface condition of the ball influences the magnitude and the reproducibility of the measured wear coefficient. Use of a polished new ball in the test may give uncertain or anomalously low wear coefficients, particularly for relatively soft specimens. This effect is explained in terms of the ease of entrainment of abrasive particles into the contact area. Reproducible results are obtained with pre‐conditioned balls with slight surface pitting, and the use of such balls is recommended for routine testing.

The abrasive and erosive wear of polymeric paint films

Abstract Two novel methods have been used to assess the wear resistance of six different 20–70 μm thick polymer coatings on both galvanized steel and polymeric substrates. The coatings were commercially available ‘clear-coats’ which are used as the uppermost layer in automobile paint systems and are intended to provide resistance to both mechanical and corrosive damage. The first wear test involves solid particle erosion and has been developed to assess the durability of monolayer thin coatings and, in this work, has been extended for the first time to multilayered systems. The second technique uses a ball rotating in a slurry of small abrasive particles to measure the abrasive wear resistance of a material. The technique allows abrasive wear tests to be performed on a sample area smaller than 4 mm2 and examines the uppermost 30 μm of material. These techniques circumvent the most common problem experienced in performing wear tests on thin coatings, namely the measurement of very small mass or volume changes. The thickness and glass transition temperature of each coating material were measured and the mechanical behaviour was characterized by tensile tests on the coating material alone. The erosion durability of the coatings was found to be sensitive to the nature of the substrate. A correlation similar to the Ratner-Lancaster correlation for bulk polymers was identified between the wear resistance of the coatings in the abrasion and erosion tests and the energy required to break the coatings in the tensile tests. The polymer films which had been formulated for use on flexible substrates were consistently more wear resistant than those designed for steel.

Resistance of paint coatings to multiple solid particle impact: effect of coating thickness and substrate material

Abstract A novel technique has been developed to determine the resistance of paint coatings to multiple solid particle impact (i.e. solid particle erosion). The effect of paint layer thickness on erosion resistance was evaluated for two acrylic automotive topcoats. These coatings displayed a two-stage response to erosion. Initially, their thickness was reduced progressively, but once a critical thickness was reached the remaining coating was removed by individual impacts. A simple model is proposed to describe this behaviour. A new measure, specific erosion resistance , which takes account of the coating thickness, is defined to allow coatings with different thicknesses to be compared and has been applied to several industrially sprayed automotive clearcoats on both steel and polymer (TPO) substrates. The clearcoats exhibited significantly higher specific erosion resistance when applied to polymer substrates.

A full factorial investigation of the erosion durability of automotive clearcoats

Abstract A full factorial experimental investigation has been carried out into factors affecting the resistance of a commercial acrylic/melamine automotive clearcoat to erosion by silica sand particles. The factor variables and their ranges were: particle size 125–425 μm; temperature 30°C–65°C; impact angle 30°–90°; particle velocity 35 m s −1 –55 m s −1 ; and the baking process applied to the coating. An empirical linear regression model for the erosion response of the coating with R 2 adj =97.5% was generated from the data. The regression coefficients of this model quantify the relative strengths of the effects of each of the factors. Several interactions between the factor variables were identified. In particular, the glass transition of the coating, which occurs at 40°C, has a significant effect on its response to erosion. The study has allowed the combinations of conditions that would be of most concern for automotive paint users to be identified.

Effect of weathering on the mechanical properties of automotive clearcoats

SummariesThe effect of weathering on the wear resistance of outomotive dearcoats has been evaluated. Acrylic-based and urethane-based coatings were exposed in Florida, Belgium and Australia and also under accelerated conditions to SAE J1960 with 0.55 W m−2 borosilicate/borosilicate filtered xenon orc light. Weathering caused a significant reduction in the abrasion and erosion resistance of the dearcoats and large increases in their hardness. Accelerated weathering produced different effects from notural exposure.RésuméL’effet de l’exposition oux intempéries sur la résistance à l’usure des revêtements clairs pour outomobiles o été évolué. Revêtements d’oayl et d’uréthonne ont été mis à l’exposition aux intempéries à Floride, en Belgique et en Australie et d’ailleurs aux conditions accélérées selon SAE J1960 avec un 0.55 W m−2 borosilicate/borosilicate filtré lampe à l’arc xenon. L’exposition aux intempéries a produit une diminution importante de la résistance à l’abrasion et à l’érosion des revêtements clairs et d’importantes croissances de leur dureté. Vieillissement accéléré produit des effets différents de ceux à partir de l’exposition aux intempéries.ZusammenfassungDer Außenbewitterungseinfluß auf die Verschleißfestigkeit von Acryl und Urethan basierten Autoklarlacke wurden in Florido, Belgien und Australien sowohl bei Kurzbewitterungsbedingungen nach SAE J1960 mit einer 0.55 W m−2 borosilikat/borosilikat durchgeschienene Xenonbogenlampe. Bewitterung wirkte eine wichtige Reduktion der Abriebs und Erosionbeständigkeit der Klarlacke und bedeutende Erhöhung deren Härte. Kurzbewitterung gibt verschiedene Effekte als Außenbewitterung.

Comments on “Micro-scale abrasive wear testing of PVD coatings on curved substrates” [by K.L. Rutherford and I.M. Hutchings]

Previously published expressions for the wear volume in the micro-scale abrasion test for curved specimen surfaces (K.L. Rutherford and I.M. Hutchings, Tribology Letters 2 (1996) 1–11) were based upon erroneous assumptions about the wear-scar geometry. Accurate volumes have now been computed, and the errors in the use of the original analytical equations are shown to be negligibly small (<0.5% error) for all practical cases.