Elsevier

Wear

Volume 258, Issues 5–6, February 2005, Pages 942-952
Wear

Micro-scale abrasive wear of silicon nitride, sialon–TiB2 composites and D2 tool steel using a multiple load method

https://doi.org/10.1016/j.wear.2004.09.049Get rights and content

Abstract

The addition of TiB2 to sialon has previously been shown to lead to improvements in tribological performance when using pin-on-disc testing. In this study the specific wear rates of Si3N4, sialon–TiB2 composites and D2 tool steel have been measured using micro-scale abrasive testing. Two variations of the micro-scale abrasive wear test have been used, free ball and fixed ball, and the results are compared. A method has been developed for the accurate measurement of the specific wear rate using multiple loads on the fixed ball apparatus. The multiple load method eliminates the dependence of the result on the accuracy of the balancing of the load arm. Scanning electron microscopy has been used to study the active wear mechanisms and the effect of increasing the applied load. The “severity of contact” model has been evaluated as a method for predicting the transition from three body rolling wear to two body grooving wear with varying load. Increasing the applied load led to slurry starvation and ridge formation before the predicted two body grooving wear could occur.

Introduction

Silicon nitride and sialon materials have been shown to have properties that make them suitable for many tribological applications. High hardness, high strength and resistance to chemical attack or corrosion are all attributes that are desirable for good tribological performance under a wide range of conditions. However, the relative brittleness of such ceramics and the expense in producing components to net shape has held back the more wide spread use of ceramics to replace metallic alloys.

Tailoring and improving the properties of silicon nitride or sialon materials can be achieved by the addition of secondary phases. In particular, the addition of particulate hard phases has been used as an approach to produce materials with increased hardness, toughness and tribological performance. The use of compounds with low resistivity such as TiN or TiB2 can produce composites that are electrically conductive [1]. Such composite materials combine the properties of ceramics with the advantage of being able to electro-discharge machine (EDM) the material to net shape without expensive diamond grinding [2].

In a previous study by the author an optimised synthesis procedure was developed for the production of sialon–TiB2 composites. At a level of 40 vol.% TiB2 the composites exhibited enhanced hardness and toughness and were electrically conductive [2], [3]. They also exhibited a dramatic decrease in the specific wear rate under like-on-like, dry sliding, pin-on-disc testing; a decrease far greater than the proportional increase in hardness or toughness [4]. No decrease in the coefficient of friction was observed and hence the improvement was attributed to the tribochemical effect of the presence of TiB2. An adherent tribofilm was formed which served to protect the underlying material from further wear.

This result was achieved using high stresses and in unlubricated conditions (load = 5 N, pin end radius ∼3 mm). Such conditions can occur in applications where lubrication may fail temporarily and these materials would hence be beneficial in such situations. However, the work described here has studied these materials under abrasive conditions in order to ascertain whether a similar improvement in wear resistance is obtained under wet, low load, abrasive conditions due to the presence of TiB2.

Two common micro-scale abrasive tests have been developed in recent years. Both tests, which are described in more detail below, use the concept of a ball rotating against the sample in the presence of an aqueous slurry of abrasive.

The first type of test is commonly known as a “free ball” test where the ball is rotated against the sample by means of friction from a rotating shaft. The clear disadvantages of this system are: the uncertainty in the number of ball revolutions due to the possibility of slip between the ball, and shaft and the load being limited to that obtained by the mass of the ball on the inclined sample. It has also been shown that non-spherical wear scars can result as the angle between ball and sample is adjusted when trying to adjust the load [5].

A more recent improvement of the free ball test is commonly known as a “fixed ball” test, where the ball is directly driven; for example, clamped between two co-axial drive shafts [6]. This method not only ensures an exact measure of the sliding distance, but also allows a range of loads to be applied using a pivoted load beam. One disadvantage of the test is that the ball rotates in a single orientation which can lead to the ball developing a flat, worn area as opposed to the free ball test where the ball generally changes orientation during the test and produces even wear over the ball surface. The fixed ball test has shown good reproducibility and is currently being studied as the basis for a new European standard for micro-scale abrasive wear [7], [8].

A recent study by Adachi and Hutchings [9] has attempted to determine the wear mode that can be expected under certain conditions when using this test. Adachi and Hutchings used a “severity of contact” model to relate the relative hardness of ball (Hb) and sample (Hs) to a quantity called the severity of contact (S), which is given asS=WAνHwhere1H=1Hb+1Hswhere W is the load (N) and v the volume fraction of the abrasive in the slurry, A the “interaction area” and is given by A = π(a2 + 2RD) in which R is the ball radius and D the abrasive particle diameter. The Hertzian contact area radius, a, is given by:a=3WR4E*1/3inwhichE*=1νb2Eb+1νs2EsandνisPoisson'sratioAdachi and Hutchings used a range of materials and test conditions to experimentally determine a relationship which predicted that, for three body rolling wear:S=WAνHαHsHbβThe constants were determined to be α = 0.076 and β = −0.49.

Using the above relationship, the properties of the materials given in Table 1 and the test conditions given in Table 2, it should therefore be possible to predict how the wear mode will change with variation in test conditions, such as increasing load or decreasing abrasive concentration. In this work only the effect of increasing load was studied.

Section snippets

Materials

Details of the synthesis of the composites and their properties have been given elsewhere [3]. A commercial hot isostatically pressed (HIP) Si3N4 material (Toshiba TSN-3NH) was used as a baseline material. It was characterised as being a low z value sialon with yttria and alumina sintering additives and a small amount of rutile (TiO2) added to give a black colour. In order to compare the tribological performance of ceramic materials with wear resistant metallic systems a D2 tool steel was

Free ball test results

The specific wear rate, κfree, was calculated for each material and Table 3 shows κfree and the standard error (Δκ) for all the materials tested. Using the free ball test the results suggest a significantly lower specific wear rate for materials containing TiB2 compared with commercial Si3N4 (TSN) and D2 tool steel. In the most extreme case Si3N4 (TSN) exhibited almost an order of magnitude higher wear rate than sialon–40 vol.% TiB2 (STB40B) while D2 tool steel exhibited the highest wear rate.

SEM examination of free ball test wear scars

As

Conclusions

  • Sufficient mixing of the abrasive slurry by magnetic stirring is only achieved by ensuring a vortex was present in the mixture. Stirring without a vortex results in the slurry settling out and a variable concentration slurry being delivered to the ball. Flat bottomed containers are necessary to form efficient vortex mixing.

  • Fixed ball testing using the multiple load method has been shown to be a more accurate method than measurements made at a single load. This method eliminates uncertainty in

Acknowledgements

The author wishes to thank Dr. Mark Gee of National Physical Laboratory, Teddington, UK for his input on the testing methods and Dr. Philip Shipman of Nottingham University for his comments on the interpretation of the wear scars. The author would also like to thank the undergraduate students Mr. Paul Roffey and Mr. Matthew Darby who carried out some of the wear testing.

References (12)

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