Friction behaviour of solid oxide lubricants as second phase in α-Al₂O₃ and stabilised ZrO₂ composites

Abstract

The influence of metal oxide additives within alumina (α-Al₂O₃) and yttria-stabilised tetragonal zirconia (Y-TZP) matrices was studied with respect to the tribological behaviour of the composites. The solid lubricants CuO, ZnO, MgO, MnO₂ and B₂O₃ were added in sufficiently small quantities (1 or 5 wt.%) to preserve the mechanical properties of the matrix phase without increasing the specific wear rate. The additives selected were an attempt to provide the materials with a self-lubricating mechanism and therefore decrease the coefficient of dry-friction. The ceramics were prepared by means of wet-ball milling powders, compaction by isostatic pressing and densification by sintering. Near-full densities (98–99%) were obtained for all ceramics except that containing CuO (92%). Tribological behaviour of the various composite compositions was tested on a pin-on-disk assembly using both α-Al₂O₃ and ZrO₂ as counter surface. Friction reductions of almost 40% for the CuO composite was obtained while the influence of the other oxide additives were less pronounced.

Introduction

Due to their high hardness, chemical inertness, high melting points, and ability to retain mechanical strength at elevated temperatures better than most metals, engineering ceramics are industrially utilised as cutting tools, a variety of bearings, biomaterials, and thermal- and corrosion-resistant coatings [1]. As structural oxide ceramics, zirconia has received much attention due to its high toughness [2], [3], [4] while alumina is known as a hard, low-cost ceramic with a high thermal conductivity [5]. Dual-phase zirconia-toughened alumina (ZTA) [6], [7], [8], [9] and ZTA-mullite [10] composites were reported as high wear-resistant materials. A major challenge in advanced structural ceramics remains to manufacture reproducible, long lifetime, self-lubricating (no second phase required) ceramic bodies for use in mechanical systems that involve high loads, velocities and temperatures. Such systems will reduce costs and be less harmful to the environment, but obtaining coefficients of friction below 0.2 is unlikely since under non-lubricated conditions current dual-phase ceramics typically has coefficients of friction of 0.5–0.8 [11]. Such high coefficients of friction generate thermal stress, which is detrimental to wear behaviour [12]. Microstructure development in the compact plays an important role in mechanical performance. A homogeneous microstructure is often obtained by the addition of oxides such as MgO, which acts as a grain inhibitor for alumina ceramics [13], or Y₂O₃, which acts as a stabiliser of the tetragonal zirconia phase [14], [15].

Wear generally increases with normal contact load, sliding distance and sliding velocities, and a wear rate of 10⁻⁶ mm³/Nm is usually set as limit above which a material is no longer considered wear-resistant. In dry-sliding wear, the minimum coefficient of friction obtained in ceramic couples is of the order 0.4 when at least one sliding partner with a high thermal conductivity is present [16], [17]. To avoid large energy losses during operation, the coefficient of friction value should ideally be <0.2, this low friction minimise heating that leads to temperature gradients in the contacting materials, which in turn result in wear initiated by fatigue processes. For example, the use of solid lubricants (such as graphite or MoS₂) intended either to protect materials from wear and corrosion or to use them at high temperatures have been described [16]. Although the addition of large volume fractions (up to 30 vol.%) of graphite lowered the friction of Y-TZP, it increased wear significantly.

Self-lubricating composites are known in literature for metals [18], [19] and increasingly more for ceramic composites [20], [21]. The preparation of alumina and Y-TZP ceramics has been described [22], [23] with various types of oxide [9], [24], [25], [26], [27] added in variable amounts. Incorporation of these oxides, however, did not target friction reduction but was used to study their influence on a variety of properties (e.g. strength, chemical stability) of the matrix ceramic phase. The addition of CuO to Y-TZP for superplastic deformation is a well-known example [22].

Porosity generally has a negative influence on the mechanical properties of a composite because pores cause stress concentrations, resulting in lower strength [28]. All composites were therefore sintered to near-full density prior to tribological testing. In this study we have addressed the tribological behaviour by forming a self-lubricating system with a specific solid-state additive in a ceramic matrix. In particular, the frictional characteristics are described for alumina and yttria-stabilised tetragonal zirconia (Y-TZP) ceramic composites with small amounts of soft oxide additives added. Reduced friction of such materials can be realised in the presence of a soft phase at the surface, which comes in contact with the opposing surface. The soft phase overcomes the difference in sliding velocity between the surfaces and shear takes place in this layer resulting in a low coefficient of friction. The major aim was reduction of friction while keeping specific wear rate very low (k_w=10⁻⁷ to 10⁻⁸ mm³/Nm). The concentration of additive was kept low (<5 wt.%) to preserve the mechanical properties of the ceramics matrix.