Toward Energy-efficient Physical Vapor Deposition : Routes for Replacing Substrate Heating during Magnetron Sputtering by Employing Metal Ion Irradiation

Sammanfattning: In this Thesis, magnetron sputtering is perfected as an environmental-friendly deposition technique. I performed systematic studies of a novel approach - hybrid high-power impulse and dc magnetron co-sputtering (HiPIMS/DCMS) with metal-ion-synchronized substrate bias pulses. The technique relies on the use of high-mass metal ion irradiation from the HiPIMS source to densify material deposited by the primary metal targets that operate in the DCMS mode. Thermally-driven adatom mobility, conventionally used to obtain high-quality layers, is replaced by low-energy recoils that are effectively created upon heavy metal ion bombardment of the growing film surface. As a result, the need for external heating is effectively eliminated and the useful growth temperature can be as low as 130 °C.   Ti-Al-N is chosen as a model materials system for the studies in this thesis due to its relevance for industrial applications and well-known challenges for phase stability control. The role of the metal ion mass on densification, phase content, nanostructure, and mechanical properties of metastable cubic Ti0.50Al0.50N-based thin films is investigated. Three series of (Ti1-yAly)1-xMexN (Me = Cr, Mo, W) films are grown with x varied intentionally by adjusting the DCMS power. There is a strong dependence of film properties on the mass of the HiPIMS-generated metal ions. All layers deposited with Cr+ irradiation exhibit porous nanostructure, high oxygen content, and poor mechanical properties. In contrast, (Ti1-yAly)1-xWxN films are fully-dense even with the lowest W concentration, x = 0.09.  A strong coupling is found between W+ incident energy Ew+ and minimum W concentration x required to grow dense (Ti1-yAly)1-xWxN layers. With lower x, higher Ew+ is needed to obtain dense films. (Ti1-yAly)1-xWxN film growth is also studied as a function of the relative Al content on the metal lattice, y = Al / (Al + Ti), covering the entire range up to the achievable solubility limit of y ~ 0.67. High-Al content films that are desired in industrial applications (as the high temperature oxidation resistance increases with increasing y) are demonstrated, while precipitation of the softer hexagonal AlN phase is avoided. It is shown that the W+ irradiation from HiPIMS source can be used to grow high-Al content layers with high hardness and low residual stress, while avoiding wurtzite AlN precipitation.  The critical parameter that controls the growth is shown to be the average momentum transfer per deposited metal adatom. W+ ion irradiation is shown to have a determining role in the densification of TiAlWN films grown by hybrid W-HiPIMS/TiAl-DCMS co-sputtering. Films with the same composition were grown as a function of the number of W+ ions per deposited metal atom, η = W+/ (W + Al + Ti). The latter was varied in a wide range by altering the peak target current density on the W target, as confirmed by time-resolved ion mass spectrometry analyses performed at the substrate plane. I demonstrate that the degree of porosity and the nanoindentation hardness are strong functions of η.   Finally, high-temperature properties of TiAlWN films grown by hybrid W-HiPIMS/TiAl-DCMS co-sputtering with no external substrate heating is explored, as motivated by application requirements, where the temperature of cutting inserts during machining exceeds 900 °C. A new age hardening mechanism was discovered with Guinier-Preston (GP) zone formation in a ceramic material. Layers with low Al content maintain high hardness well above the annealing temperature characteristic of spinodal decomposition. The evidence from electron microscopy, ab initio calculations, and molecular dynamics simulations, shows that the GP effect originates from the formation of atomic-plane-thick W discs populating {111} planes of the cubic matrix. The results demonstrate for the model materials system of TiAlN that the process energy consumption can be reduced by as much as 64% with respect to conventional methods, with no compromise on coating quality. 

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