Application of in situ Transmission Electron Microscopyfor Tribological Investigations of Magnetron Sputter Assisted Pulsed Laser Deposition of Yttria-stabilized Zirconia-gold Composite Coatings

Abstract

Yttria-stabilized zirconia (YSZ)-Au composite coatings have great potential as solid film lubricants for aerospace applications over a wide range of environmental conditions. They were grown on steel disks or silicon wafers by pulsed laser ablation of YSZ and simultaneous magnetron sputtering of a Au target. Such a combination of ceramics with soft metals improved the toughness of the composite coating and increased its ability to lubricate at high temperature. Information on the time-dependent response of these microstructures to changes in temperature is essential to tribological investigations of high temperature performance. In situ transmission electron microscopy was used to directly measure the dynamic change of YSZ-Au coating structure at elevated temperatures. High-resolution electron microscopy and electron diffraction showed that amorphous YSZ-5 at.% Au coatings proceeded to crystallize under the irradiation of electron beams. Time varying x-ray energy dispersive spectra measured a loss of oxygen in the sample during about 10 min of irradiation with subsequent slight oxygen recovery. This behavior was related to the activation of oxygen diffusion under electron irradiation. X-ray diffraction patterns from vacuum annealed samples verified crystallization of the coatings at 500°C. Real-time growth of Au nanograins in the sample was observed as the temperature was increased to 500°C in a TEM specimen holder that could be heated. The grain growth process was recorded using a charge-coupled device camera installed on the transmission electron microscope. The crystallization and growth of zirconia and Au nanograins resulted in low friction during tribological tests. The nucleation of Au islands on heated ball-on-flat specimens was responsible for lowering friction.

Preparation of chameleon coatings for space and ambient environments

“…Broad peaks corresponding to Au are evident for both coatings. For the low Au sample with 59% YSZ the (111) ZrO 2 peak is evident at 2θ ≈ 30°, which gives a close match to the P4m2 tetragonal symmetry with lattice parameters a = 0.512 nm and c = 0.525 nm [15,16]. This sample represents the maximum in YSZ obtained for all coatings in this study.…”

Silver diffusion and high-temperature lubrication mechanisms of YSZ–Ag–Mo based nanocomposite coatings

“…Yttria-stabilized zirconia (YSZ) based nanocomposite coatings have received growing attention for their remarkable properties such as high hardness, toughness, wear resistance and low friction [1][2][3][4][5][6][7][8][9][10][11][12][13]. These properties are highly desirable for aerospace applications in order to increase the lifetime and performance of mechanical systems.…”

“…Proper sized nanocrystalline grains can restrict crack size, create a large volume of grain boundaries, and hence improve the coating toughness and contact load support [24,25]. The nanocomposite coatings consisting of YSZ ceramics and noble metals were initially studied for tribological uses at high temperatures [6][7][8][9].…”

“…The weak bonding between ceramic and metallic components can facilitate the grain boundary sliding and improve the composite toughness. The in-situ transmission electron microscopy studies of YSZ-Au nanocomposites during heating showed that the diffusion process was accompanied by the coating microstructural and chemical changes [9]. Increased temperatures initiated the crystallization and growth of zirconia and gold nanograins inside the coatings and formation of 100-500 nm gold islands on the coating surfaces.…”

Multilayered YSZ–Ag–Mo/TiN adaptive tribological nanocomposite coatings

“…Chameleon coatings consisting of various nanoinclusions imbedded in an amorphous YSZ-noble metal matrix have demonstrated low friction coefficients at elevated temperatures resulting from noble metal diffusion to the coating surface [2,4,9]. Recent in-situ TEM studies of YSZ-Au nanocomposites during heating showed that the noble metal diffusion process is accompanied by coalescence at the surface [10]. From work on powder-based and other types of composites, it is known that silver is an effective lubricant up to moderately high temperatures [11][12][13][14][15][16].…”