Superior wear resistance of diamond and DLC coatings

Abstract

As the hardest known material, diamond and its coatings continue to generate significant attention for stringent applications involving extreme tribological conditions. Likewise, diamond-like carbon (DLC, especially the tetragonal amorphous carbon, ta-C) coatings have also maintained a high level interest for numerous industrial applications where efficiency, performance, and reliability are of great importance. The strong covalent bonding or sp³-hybridizaiton in diamond and ta-C coatings assures high mechanical hardness, stiffness, chemical and thermal stability that make them well-suited for harsh tribological conditions involving high-speeds, loads, and temperatures. In particular, unique chemical and mechanical nature of diamond and ta-C surfaces plays an important role in their unusual friction and wear behaviors. As with all other tribomaterials, both diamond and ta-C coatings strongly interact with the chemical species in their surroundings during sliding and hence produce a chemically passive top surface layer which ultimately determines the extent of friction and wear. Thick micro-crystalline diamond films are most preferred for tooling applications, while thinner nano/ultranano-crysalline diamond films are well-suited for mechanical devices ranging from nano- (such as NEMS) to micro- (MEMS and AFM tips) as well as macro-scale devices including mechanical pump seals. The ta-C coatings have lately become indispensable for a variety of automotive applications and are used in very large volumes in tappets, piston pins, rings, and a variety of gears and bearings, especially in the Asian market. This paper is intended to provide a comprehensive overview of the recent developments in tribology of super-hard diamond and DLC (ta-C) films with a special emphasis on their friction and wear mechanisms that are key to their extraordinary tribological performance under harsh tribological conditions. Based on the results of recent studies, the paper will also attempt to highlight what lies ahead for these films in tribology and other demanding industrial applications.

Introduction

Naturally occurring bulk diamond and its crystalline coatings remain as the hardest material known while DLC coatings (especially ta-C with very high sp³ content) are structurally disordered but can also provide exceptional hardness (i.e., 70–90 GPa) and stiffness comparable to those of diamond and other superhard materials [1], [2], [3], [4]. Such highly impressive mechanical properties of diamond and ta-C DLC coatings are thought to stem from their extreme atomic density with shortest bond lengths and strong covalent (sp³) bonding between the carbon atoms [3], [4], [5]. Fig. 1 shows crystalline and disordered atomic configurations of diamond and typical ta-C structures.

These coatings are very resistant to biological, chemical or corrosive attacks by almost all known acids and bases and are inherently very resistant to abrasive, adhesive, and erosive wear [6], [7]. Therefore, they are ideal candidates for applications involving harsh tribological conditions involving open air, hydrogen, liquid lubrication, moderate temperatures and sliding velocities [8], [9]. However, under high temperatures and in vacuum or inert gas environments, they may suffer elevated friction and wear losses due to highly adhesive covalent bond interactions at their rubbing interfaces [6], [9], [10], [11], [12]. Overall, passivation of covalent bonds with hydrogen, hydroxyl group and/or other gaseous species as well as the formation of an amorphous or graphitized tribolayer appear to dominate friction and wear of diamond and ta-C films.

There now exist numerous physical and chemical vapor deposition (PVD and CVD) methods for the production of DLC and diamond coatings on all kinds of substrate materials [13], [14], [15], [16], [17], [18]. All of them involve gas discharge plasmas primarily consisting of hydrocarbon gases (as the carbon source) and hydrogen and/or argon. In the case of DLC, solid carbon targets such as graphite and amorphous or glassy carbons may also be used for the extraction of carbon for ta-C film growth. Deposition conditions are tailored to dissociatively extract carbon from the gaseous species and produce highly energetic ionic and neutral carbon species that are necessary for the initial nucleation and subsequent growth of these films. Among all other hard coatings, diamond and DLCs have gained huge industrial acceptance and use in recent years [14], [19], [20]. Some of the industrial applications include nano/microelectromechanical systems, or NEMS/MEMS [21], [22], magnetic hard disks [23], cutting tools [24], all kinds of automotive parts and components [13], [25], mechanical seals [26], biomedical implants [27], [28], and scratch-resistant optical windows [29].

Since their initial inception for more than a half Century ago [30], [31], countless research articles, conference proceedings, books, and handbook chapters have been devoted to diamond and DLC coatings [13], [14], [15], [16], [17], [18], [19]. Such a vast knowledge base was very instrumental in further optimizing/customizing these coatings to meet the increasingly more stringent operating conditions of many tribological applications. In this paper, we provide a comprehensive overview of recent developments in the tribology of diamond and ta-C DLC films with emphasis on their extraordinary wear performance under dry and lubricated sliding conditions. Also provided is the present state of the art in the making of smooth diamond films with nano-to-ultra-nano grain sizes giving some of the lowest friction and wear coefficient. As mentioned, diamond and ta-C films are used extensively in combatting friction and wear especially in tooling and automotive applications where wear of moving parts became a major issue due to increasingly harsher operating conditions. However, in this paper, we mostly concentrate on the wear of these coatings and specifically highlight the fundamental mechanisms that control their exceptional wear performance. We will also touch on the deposition conditions that can lead to the synthesis of smooth diamond and DLC films that are most suitable for superlow-friction and -wear in sliding tribological applications.