W.-D. Münz

Influence of high power densities on the composition of pulsed magnetron plasmas

The application of high power pulses with peak voltage of -2 kV and peak power density of 3 kWcm-2 to magnetron plasma sources is a new development in sputtering technology. The high power is appli ...

High power pulsed magnetron sputtered CrNx films

Microstructure and macroscopic properties of droplet free CrN films deposited by the recently developed high power pulsed magnetron sputtering (HIPIMS) technique are presented. Magnetron glow discharges with peak power densities reaching 3000 W cm−2 were used to sputter Cr targets in both inert and reactive gas atmospheres. The flux arriving at the substrates consisted of neutrals and ions (approx. 70/30) of the sputtered metal and working gas atoms (Ar) with significantly elevated degree of ionization compared to conventional magnetron sputtering. The high-speed steel and stainless steel substrates were metal ion etched using a bias voltage of −1200 V prior to the deposition of CrN films. The film-to-substrate interfaces, observed by scanning transmission electron microscope cross-sections, were clean and contained no phases besides the film and substrate ones or recrystallized regions. CrN films were grown by reactive HIPIMS at floating potential reaching −160 V. Initial nucleation grains were large compared to conventional magnetron sputtered films, indicating a high adatom mobility in the present case. The films exhibited polycrystalline columnar growth morphology with evidence of renucleation. No intercolumnar voids were observed and the corrosion behavior of the film was superior to arc deposited CrNx. A high density of lattice defects was observed throughout the films due to the high floating potential. A residual compressive stress of 3 GPa and a hardness value of HK0.025=2600 were measured. A low friction coefficient of 0.4 and low wear rates against Al2O3 in these films are explained by the absence of droplets and voids known to contribute to extensive debris generation.

Microstructure and oxidation-resistance of Ti1-x-y-zAlxCryYzN layers grown by combined steered-arc/unbalanced-magnetron-sputter deposition

Abstract Cation-substituted Ti 1 − x − y −z Al x Cr y Y z N alloys, with y = 0.03 and z = 0.02, have been shown to offer greatly enhanced high-temperature oxidation resistance compared to presently used TiN and Ti 1 − x Al x N films. Layers (3 μm thickness) were deposited by unbalanced magnetron sputter deposition onto austenitic stainless steel and M2 high-speed steel substrates which had been ion etched in situ using a steered Cr-metal-ion cathudic arc discharge at an Ar pressure of 6 × 10 −4 mbar (0.45 mTorr). The metal ion-etching promoted initial local epitaxy on individual substrate grains while the overall film texture evolved through competitive growth to (111) in Ti 0.44 Al 0.53 Cr 0.03 N alloys and (200) in Ti 0.43 Al 0.52 Cr 0.03 Y 0.02 N. Although Ti 0.44 Al 0.53 Cr 0.03 N layers exhibited a columnar microstructure similar to that previously observed in Ti 1 − x Al x N alloys, the addition of 2 mol% YN resulted in significant grain refinement giving rise to a more equiaxed structure. The Knoop microhardness of Ti 0.43 Al 0.52 Cr 0.03 Y 0.02 N alloys was HK 0.025 = 2700 kg mm −2 compared to 2400 kg mm −2 for Ti 0.44 Al 0.53 Cr 0.03 N. The onset of rapid oxidation, as determined from thermo-gravimetric measurements, ranged from ≈ 600 °C for TiN to 870 °C for Ti 0.46 Al 0.54 N to 920 °C for Ti 0.44 Al 0.53 Cr 0.03 N to 950 °C for Ti 0.43 Al 0.52 Cr 0.03 Y 0.02 N.

Raman microscopic studies of PVD hard coatings

Abstract A series of PVD ceramic hard coatings (TiN, ZrN, TiAlN, TiZrN and TiCN) were deposited on steel substrates using the cathodic arc/unbalanced magnetron deposition technique. These coatings were characterised using Raman microscopy to elucidate the behaviour of the optic and acoustic phonon modes of the (cubic) crystalline lattices. Defect-induced first- (and second-) order spectra have been observed in the 200–300 and 500–800 cm −1 regions and these have been assigned and correlated with coating composition. Changes in the position, intensity and shape of the principal TO band (640–560 cm −1 ) have been interpreted. Raman microscopy has been shown to be a very useful non-destructive complementary technique to XRD for the characterisation of PVD hard coatings.

Use of an externally applied axial magnetic field to control ion/neutral flux ratios incident at the substrate during magnetron sputter deposition

The development and characterization of an ultrahigh vacuum ‘‘unbalanced’’ dc magnetron sputter deposition system with a variable external axial magnetic field for controlling the ion‐to‐neutral flux ratio at the substrate during deposition with low negative substrate biases is reported. The target assembly is a planar‐magnetron (PM) with a toroidal magnetic‐field electron trap created using a set of permanent magnets. A pair of Helmholtz coils, located outside the vacuum chamber, produces an additional magnetic field Bext which is uniform along the axis orthogonal to both target and substrate surfaces. The value and sign of Bext has a strong effect on the plasma density near the substrate, and hence on the ion flux Ji incident at the substrate, with only a minor effect on the target‐atom flux. For a Ti target sputtered in pure Ar at 20 mTorr with a target‐substrate separation of 6.5 cm, changing Bext from −50 G (opposing the field of the outer PM pole) to +600 G (reinforcing the field of the outer PM pol...

Low‐energy (∼100 eV) ion irradiation during growth of TiN deposited by reactive magnetron sputtering: Effects of ion flux on film microstructure

Cross‐sectional transmission electron microscopy (XTEM) has been used to investigate the effects of variations in the low‐energy ion irradiation flux during the growth of reactively sputter‐deposited TiN. The films were deposited on steel substrates with a negative bias of 100 V at 350 °C in mixed Ar–N2 discharges at a pressure of 5 Pa (37 mTorr). The ion‐to‐Ti arrival rate ratio Jion /JTi at the substrate was varied between 0.3 and 7.1 through the use of a variable external magnetic field. Films grown with Jion /JTi ≲2 had a columnar morphology with a highly underdense microstructure. Increasing Jion /JTi ≳4 was sufficient to cause the growth of dense films with a more equiaxed grain structure due to renucleation. Further increases in Jion /JTi ≳7 resulted in increased TiN grain size and local heteroepitaxy between TiN and the martensitic phase of the substrate.

Microstructures of TiN films grown by various physical vapour deposition techniques

Abstract A study has been made of TiN coatings deposited on steel substrates by five commercially available physical vapour deposition (PVD) methods; low voltage electron beam evaporation, triode high voltage electron beam evaporation, random-arc evaporation, steered-arc evaporation and magnetron sputtering. The microstructure and substrate-film interfacial microchemistry of the films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) together with energy-dispersive X-ray spectroscopy (EDX), cross-sectional transmission electron microscopy (XTEM) and scanning transmission electron microscopy (STEM) combined with EDX analyses of XTEM samples. The XRD analyses showed that all the films were in a state of compressive stress with interplanar distances as much as 1.7% higher than reference bulk values. SEM examination revealed only minor variations in surface roughness among the samples except for the arc-evaporated films which contained large droplets and craters resulting from the detachment of droplets. The number density and average sizes of droplets and craters were lower in the steered-arc sample than in the random-arc sample. XTEM analyses showed that all the films had columnar structures with clearly defined substrate-film interfacial layers. The films appeared dense except for the magnetron-sputtered sample which exhibited intercolumnar porosity. STEM-EDX analyses showed large variations in the microchemistry of the substrate-film interfacial regions which consisted, depending on the sample, of renucleated near-surface substrate grains, intentionally (or, in at least one case, unintentionally) introduced foreign material or gas-bubble-like inclusions. However, the microchemistry of these interfacial regions was, in most cases, understandable on the basis of the substrate pretreatment and/or choice of film growth parameters.

Ion-assisted growth of Ti1−xAlxN/Ti1−yNbyN multilayers by combined cathodic-arc/magnetron-sputter deposition

Abstract The microstructure and microchemistry of polycrystalline Ti1−x−yAlxNbyN alloys and Ti1−xAlxN/Ti1−yNbyN multilayers grown on bee ferritic stainless steel substrates at 450 °C by unbalanced-magnetron (UBM) sputter deposition and combined UBM/cathodic-arc (UBM/CA) deposition have been investigated using X-ray diffraction, scanning electron microscopy, Rutherford backscattering spectrometry, cross-sectional transmission electron microscopy (XTEM), and scanning transmission electron microscopy with energy-dispersive X-ray microanalyses of XTEM samples. The growth experiments were conducted in a deposition system in which the substrates are continuously rotated past one Ti0.85Nb0.15 and three Ti0.50Al0.50 cathodes, each capable of being operated independently in either UBM or CA mode. CA ion-etching of the steel substrates prior to deposition produced a polycrystalline compositionally-graded altered layer with a depth of ⋍ 20 nm when operating the arc on the Ti0.50Al0.50 target and a much narrower, ⋍ 6 mm. amorphous layer with the Ti0.85Nb0.15 target. Subsequent UBM or UBM/CA growth on substrates subjected to the former treatment resulted in local epitaxy with underlying grains for film thicknesses up to ⋍ 200 nm before the growth front gradually broke down locally to initiate columnar deposition. Film growth on substrates CA ion-etched using the Ti0.85Nb0.15 target resulted in much smaller average column diameters with competitive grain growth. However, the fraction of the sample area covered by nodular defects arising from arc-ejected droplets was reduced by a factor of ⋍ 102. Thus the latter procedure was used for substrate preparation prior to multilayer growth. Ti1−xAlxN/Ti1−yNbyN multilayers with periods between 2.17 and 2.29 nm were grown by combined UBM/CA deposition. The multilayers exhibited flat, regular interfaces throughout film total thicknesses of up to 3 μm.

Thermal stability and oxidation resistance of TiAlN/CrN multilayer coatings

Abstract TiAlN/CrN multilayer films with superlattice characteristics were deposited onto M2 high-speed steel and 304 stainless steel by reactive unbalanced magnetron sputtering involving three TiAl targets and one Cr target. The wavelength of the multilayer structures (ranging from 2.4 nm to 4.8 nm) and Cr content (from 10% to 30%) was controlled by varying the power supplied to the Cr target (P Cr ) from 2 kW to 12 kW. These coatings were then heated in an ambient atmosphere at temperatures ranging from 500 to 1000 °C and for times up to 16 h. Breakdown of the multilayer structure was studied using such techniques as hardness measurements, X-ray diffraction (XRD) and cross-sectional TEM (XTEM). The results show that both the multilayer structure and hardness can be maintained for as long as 16 h at 700 °C. This was found to be independent of the Cr content (multilayer wavelength) of the coatings. Oxidation behaviour involving isochronal thermogravimetric analysis (TGA), XRD and SEM revealed that all coatings exhibited weight gains of as little as 5% of that of TiN under similar conditions at 1000 °C. The resistance to oxidation was found to be dependent on the Cr content, with the greatest effect occurring in the P Cr = 12 kW samples. Two very distinct regimes of oxidation were observed. Up to approximately 900 °C, the rate of oxidation was very low (approximately 0.2 gm 2 in the case of the P Cr = 4 kW samples and 0.1 gm −2 in the case of the P Cr = 12 kW samples). Above this temperature, the oxidation rates increased markedly for all samples, with that of the P Cr = 12 kW being the least severe.

The corrosion behaviour of macroparticle defects in arc bond-sputtered CrN/NbN superlattice coatings

The investigation concerned the corrosion behaviour of macroparticle and growth defects in PVD CrN/NbN superlattice coatings formed by are bond-sputtering (ABS) process on a mild steel BS6323. The electrochemical behaviour of the coatings was firstly studied by potentiodynamic polarising in de-aerated 0.5 M (Na2CO3-NaHCO3) buffer and 5% NaCl solutions, respectively. The coating and defects were then examined in planar view and cross-section by scanning electron microscopic analyses and the results were compared with those prior to the electrochemical measurement. It is found that the overall coating/substrate corrosion process is closely related to the deleterious effect of the macroparticles and growth defects in the PVD coatings. It is further demonstrated that for through thickness macroparticle inclusions, corrosion initiates by galvanic or crevice corrosion between the defect and the coating matrix, subsequently permitting solution access to those defects with eventual substrate pitting and corrosion at the coating/substrate interface. On the basis of the experimental findings and the macroparticle formation theory, the mechanisms of the growth defect-related coating/substrate corrosion are finally proposed