Nanostructured Thin Films Nanodispersion Strengthened Coatings PDF
Preview Nanostructured Thin Films Nanodispersion Strengthened Coatings
Chapter 1 SMART NANOCOMPOSITE COATINGS WITH CHAMELEON SURFACE ADAPTATION IN TRIBOLOGICAL APPLICATIONS A. A. Voevodin and J. S. Zabinski Air Force Research Laboratory, Materials and Manufacturing Directorate Wright Patterson Air Force Base, OH 45433-7750, U.S.A. Smart nanocomposite tribological coatings were designed to respond to changing environmental conditions by self-adjustment of their surface properties to maintain good tribological performance in any environment. These coatings have been dubbed “chameleon” because of their ability to change their surface chemistry and structure to avoid wear. The first “chameleon” coatings were made of WC, WS2, and DLC; these coatings provided superior mechanical toughness and performance in dry/humid environmental cycling. In order to address temperature variation, the second generation of “chameleon” coatings were made of yttria stabilized zirconia (YSZ) in a gold matrix with encapsulated nano-sized reservoirs of MoS2 and DLC. High temperature lubrication with low melting point glassy ceramic phases was also explored. All coatings were produced using a combination of laser ablation and magnetron sputtering. They were thoroughly characterized by various analytical, mechanical, and tribological methods. Coating toughness was remarkably enhanced by activation of a grain boundary sliding mechanism. Friction and wear endurance measurements were performed in controlled humidity air, dry nitrogen, and vacuum environments, as well as at 500-600 ºC in air. Unique friction and wear performance in environmental cycling was demonstrated. Keywords: nanostructured materials, composite, coating, hard, tough, low friction, wear reduction, environment cycling. 1. INTRODUCTION Nanocomposite coatings offer a unique opportunity to design and produce adaptive or smart tribological coatings, which were termed “chameleon” for their ability to resist friction and wear by changing surface chemistry and microstructure in response to environment and loading changes [1,2], much like a chameleon changing its skin color to avoid predators. Although quite challenging, practical realization of smart coatings is extremely rewarding for tribological pairs subjected to multiple environmental changes, as for example in aerospace applications. The first 1 A. A. Voevodin et al. (eds.), Nanostructured Thin Films and Nanodispersion Strengthened Coatings, 1–8. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 2 realization of such coating growth was made using a mixture of oxides and dichalcogenides (PbO/MoS2, CFx/WS2, ZnO/WS2), which could operate in a broad range of temperatures [3-5]. Advanced multilayer structures were then designed to combine these composites with buried diffusion barrier layers and achieve surface self-adaptation during repeated temperature cycling. Recently, novel wear resistant materials were developed, which combine nanocrystalline carbides (TiC, WC), oxide based ceramics (YSZ and AlON), dichalcogenides (MoS2, WS2), and amorphous diamond-like carbon (DLC) into nanocomposite structures [1,2,6]. For example, “chameleon” coatings made of amorphous diamond-like carbon (DLC) matrix with incorporation of nanocrystalline TiC [7,8], WC [9,10], WS2 [1,11] and laser processed MoS2 reservoirs [12] had demonstrated an order of magnitude improvement in toughness above that of single phase carbides while maintaining the same level of hardness, a low friction coefficient in cycling from dry to humid environments, and an extremely long life in both ambient and space environments. The surface chemistry, structure, and mechanical behavior of these nanocomposite materials were shown to reversibly change in the tribological contact, depending on applied loads and operational environment to maintain low friction and prevent wear. While keeping the low friction in any environment is important, the coating wear resistance requires an additional blend of both hardness and fracture toughness. Nanocomposite coating designs provide a unique opportunity to achieve the best blend of the toughness, wear resistance, and a low friction performance across multiple environments. This paper reviews the most recent developments in smart nanocomposite tribological coatings, starting with design criteria and examples of tough tribological nanocomposites and progressing to the design and examples of “chameleon” coatings. 2. TOUGH NANOCOMPOSITE TRIBOLOGICAL COATINGS An approach with embedding grains of a hard, high yield strength phase into a ductile matrix has been widely explored in macro-composites made of ceramics and metals which are known as cermets [34]. However, if grain sizes and matrix separations are reduced to a nanometer level, dislocation activity as a source of ductility is eliminated and new mechanisms are required to provide toughness enhancement. Uniquely, these nanocomposites contain a high volume of grain boundaries with a crystalline/amorphous transition across grain-matrix interfaces, limiting initial crack sizes and helping to deflect and terminate growing cracks. These mechanisms helped to explain the fracture resistance of novel super- hard composites, where both amorphous ceramic [13-15] and metal [16-18] 3 matrices were used to encapsulate nanocrystalline ceramic grains. More on the super-hard nanocomposite coating design and discussion of the involved mechanisms can be found in available review articles [14,15,17,19,20]. Grain boundary diffusion [21] and grain boundary sliding [21-25] were suggested as mechanisms for improving ductility and providing super- plasticity of single phase ceramic nanocrystaline systems. The most recent research indicates also that high ductility can be more easily achieved in multiphase structures [26] and that grain Figure 1. Schematic of a tough nanocomposite boundary sliding is a primary coating design, combining a nanocrystalline / mechanism of super-plasticity amorphous structure with a functionally [27-30]. It was also found that gradient interface. equiaxial grain shapes, high angle grain boundaries, low surface energy, and the presence of an amorphous boundary phase facilitate grain boundary sliding [21,22]. These findings can be expanded into the field of hard wear resistant coatings to introduce ductility and prevent fracture under a high contact load. In the course of the development of tough tribological nanocomposite coatings the following design concepts were used: (1) a graded interface layer is applied between the substrate and crystalline/amorphous composite coating to enhance adhesion strength and relieve interface stresses (combination of functional gradient and nanocomposite designs) [31-33]; (2) encapsulation of 3-10 nm sized hard crystalline grains in an amorphous matrix restricts dislocation activity, diverts and arrests macro- crack development, and maintains a high level of hardness similar to super- hard coating designs [14,17]; (3) a large volume fraction of grain boundaries provides ductility through grain boundary sliding and nano-cracking along grain/matrix interfaces [8,34,35]. These design concepts are targeting toughness enhancement through stress minimization, crack deflection, and ductility introduction. They are close but still differ from the design concepts of superhard nanocomposite coatings. The primary differences are viewed in the selection of a matrix phase with a lower elastic modulus, relaxation of the requirement for strong binding between matrix and grains, and selection from a greater range of acceptable grain sizes of nanocrystals phase embedded in an amorphous matrix phase. 4 Combination of the nanocrystalline/amorphous designs with a functionally graded interface is shown in Figure 1. This design provides both high cohesive toughness and high interface (adhesive) toughness in a single coating. Several examples of tough wear resistant composite coatings have been produced. Two of them combined nano- crystalline carbides with an Figure 2. TEM image of an YSZ/Au nanocomposite coating with improved toughness amorphous DLC matrix characteristics designated as TiC/DLC and WC/DLC composites. In another example, nanocrystalline YSZ grains were encapsulated in a mixed YSZ-Au amorphous matrix as shown in Figure 2. In all cases, the large fraction of grain boundary phase provided ductility by activating grain boundary slip and crack termination by nanocrack splitting. This provided a combination of high hardness and toughness in these coatings. Figure 3, compares Vickers indentations made at the highest load of the machine (1 kg). There are no observable cracks in these coatings, even after significant substrate compliance (indentation marks are 9 Pm deep into 1 Pm thick coatings). The coating hardness was quite high ranging from 18 to 30 GPa, and for most other typical hard coatings at these loads, cracks in the corners of the indentations are expected. Thus, novel nanocomposite designs of tough tribological coatings are very promising. They explore fundamentally different concepts of the toughness improvement, when compared to macrocomposite materials such a) b) c) Figure 3. Knoop and Vickers indenation marks into the surface of 1 Pm thick (a) TiC/DLC, (b) WC/DLC, and (c) YSZ/Au tough nanocomposite coatings. Indents were performed with the maximum available load of 1000 gm, providing about 9 Pm indentation depth due to the steel substrate deformation. In all cases, there were no cracks at the indention corners, which serve as stress risers. 5 as cermets. Furthermore, their design can be taken to the next level by realizing that both matrix and nanograins can serve as reservoirs of solid lubricants. These lubricants will be then released in a friction contact in the course of sliding. The active tribological role of matrix and encapsulated grains is a basis for the tough and low friction “chameleon” coatings, which are discussed in the next section. 3. “CHAMELEON” TRIBOLOGICAL COATINGS The objective of “chameleon” tribological coating designs is to provide a reversible self-adjustment of surface chemistry, structure, and mechanical behavior in the friction contact, depending on applied loads and operational environment to maintain low friction and prevent wear. In order to achieve reversible adaptation, two additional design concepts were developed and combined with the tough nanocomposite coating concepts (design concept numbering is continued here from Section 2): (4) solid lubricant reservoirs are introduced as amorphous or poorly crystalline inclusions to minimize reduction in composite hardness and elastic modulus, since crystalline solid lubricants are typically very soft [1,36]; (5) friction forces and surface reactions with the environment are used to generate a lubricious transfer film or “skin” at the tribological contact, which can self-adjust with each environmental change [1,36]; i.e,. coating components serve as reservoirs to supply material for the “tribo-skin”, where formation of a lubricating film with the required chemistry and structure reduces friction. Figure 4 presents a schematic of a nanocomposite coating design that exhibits “chameleon” behavior. This design was implemented in the fabrication of the YSZ/Au/DLC/MoS2 and WC/DLC/WS “chameleon” 2 coatings where an amorphous matrix and a hard nanocrystalline phase (e.g., YSZ or WC) were used to produce optimum mechanical performance and load support. Nanocrystaline and amorphous Au, MoS2, and Figure 4. Schematic of a conceptual design of the YSZ/Au/MoS2/DLC tribological coating with DLC were added to achieve chameleon-like surface adaptive behavior. chemical and structural
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