ICMCTF2003 Session E1-2: Friction and Wear of Coatings

Erosion and wear of materials in corrosive environments occur in many diverse industries, ranging from power generation to offshore environments. The extent of wastage is dependent on the materials involved in the tribo-contact and any corrosion product arising from the tribological action and the environment. In such cases, there is said to be either a "synergy" or "antagonism" between the processes, where the corrosion product can either enhance, or inhibit, the overall wastage rate.

A significant advance in recent years has been the construction of engineering maps to characterise tribo-corrosion processes. Such maps may identify the mechanism of damage and the extent of wastage. More recently, the extent of "synergism" and "antagonism" between the processes has been delineated on such maps, thus providing a powerful tool of highlighting regions where corrosion may have a significant effect on the overall wastage rate.

This paper describes recent mapping research for particulate erosion-corrosion of a range of PVD coated steels. The rationale for identification of wastage mechanisms and the extent of "synergy"/"antagonism" is described, and the application of such mapping approaches to other tribo-corrosion processes (such as sliding wear-corrosion and abrasion-corrosion) is also addressed. Future work on the development of theoretical mapping approaches for coated systems in tribo-corrosion processes is discussed, in terms of the range of possible variables involved, and the complexities of the individual tribo-chemical interactions.

Significant progress has been made in recent years on the study of micro-abrasion mechanisms of materials. Regimes of micro-abrasion have been proposed which identify whether the wear occurs either by a two body, or three body mechanisn. Micro-abrasion maps have been constructed showing the variation in wear regime, as a function of applied load and velocity.

There has, however, been little work carried out to date on the micro-abrasion of composite coatings in such conditions. Here, because the size of the contact zone of the abrasion event may approach that of the abradent and the reinforcement (both typically less than 10 µm), the wear mechanism is complex. In such cases, the properties of the reinforcement and the matrix material of the coatings may have a significant effect on the overall wastage rate.

In this study, the micro-abrasion resistance of a range of WC/Co based coatings was studied in aqueous media. Micro-abrasion mechanism maps were constructed indicating the wear regimes in such conditions. Possible reasons for the variation of the boundaries of the maps for the different WC/Co based coatings are outlined, with reference to the mechanical and chemical properties of the individual coatings exposed the abrasive environment.

Wear maps have been used successfully to show the complexity of wear and the effect of speed and load on wear mechanisms. Wear mechanism maps allow various wear equations to be developed to describe wear in that particular regime. The effect of lubricants, wear transitions can be shown clearly for a system. Various wear mapping techniques will be discussed and compared on different materials.

Wear mapping on coatings and thin films have additional complications in residual stress, coating thickness, and delamination at the coating-substrate interface. How these parameters can be handled in wear maps require additional treatment. This paper will show the current state of the art in wear mapping, the techniques used to construct wear maps, and special accommodations to coatings and thin films.

This paper treats the importance and benefits of making an optimised selection of substrate and surface preparation when applying thin (1 - 10 µm) coatings on tools and mechanical components. Different aspects of the selection are discussed, examples of good and bad choices are given and some general recommendations are presented.

The selection of substrate material will be discussed in terms of chemical composition, hardness, size and distribution of second phases, in relation to the properties of the counter material. Generally, metal cutting and hot-forming applications demand higher hardness and better thermal resistance of the substrate, but are less demanding with respect to contact fatigue than are lubricated machine components such as gears, bearings, seals, etc.

The selection of surface preparation is discussed in terms of topography achieved and possible impairment of the substrate structure and hardness. Normally the substrate surface of both tools and mechanical components should be made as smooth as possible, while avoiding high temperatures in the preparation process which could result in annealing and loss of hardness. It will be demonstrated that the critical load for coating failure in scratch testing and in sliding contacts increases with reduced roughness down to an R_a value of about 0.1 µm. The benefits of simple post polishing of the coating surface will be demonstrated for different application types. The mechanisms of material pick-up and increased friction due to protruding asperities on tool and component surfaces will be demonstrated and discussed.