ICMCTF2014 Session B1-3: PVD Coatings and Technologies

The composition, microstructure, residual stress and hardness of ZrO₂-Al₂O₃ coatings deposited by pulsed-DC magnetron sputtering (PDCMS) were compared to those prepared by the filtered vacuum arc (FVAD) technique. The coatings were obtained on Si and WC substrates at various substrate temperatures (T_s = 300-850 K), substrate bias (V_b, between floating and -200 V), and Zr:Al power source ratio.

The PDCMS deposition rate was ~10 nm/min, while that of FVAD was higher, 100-200 nm/min as a function of V_b and T_s. By controlling the Zr:Al power source ratio, the PDCMS coatings were deposited over a broad range of compositions, i.e. Zr:Al ratio from 9:1 up to 1:9. However, due to arc stability limitations, the Zr:Al ratio of the FVAD coatings varied between 1:1 and 5:1. It was found that PCDMS coatings deposited with a Zr:Al ratio of 1:9 to 3:7 exhibits a composite structure with a stabilized ZrO₂ crystalline phase and an amorphous Al₂O₃ phase, while those deposited at a higher Zr:Al ratio had an amorphous (X-ray amorphous) structure. Similar results were observed for FVAD depositions, namely crystalline structure for Zr:Al = 5:1 and amorphous for 1:1. XRD analysis indicated that PDCMS coatings had stabilized cubic-ZrO₂ structures as T_s increased, or tetragonal-ZrO₂ as V_b increased. The structure of the FVAD coatings could not be determined because the diffraction patterns were very broad.

Thick Al2O3 coating deposited by thermal CVD is still dominantly used for high speed turning of cast iron, mainly because of superior stability at high temperature. There are some concerns about thermal CVD process, however, such as safety issue coming from somewhat hazardous gaseous sources. PVD process has advantages over CVD process in terms of easy operation and variety of coating material. Generally, hard coatings deposited by PVD process with highly ionized plasma are characterized by high degree of compressive stress and due to this stress, maximum coating thickness is usually limited less than 10 microns or even 5 microns on the sharp cutting edge. This has been made PVD coating not suitable for applications where certain coating thickness is mandatory.

Kobe Steel has developed a new type of arc cathode which is able to control the residual stress in a wide range and this made it possible to produce thick coating without chipping or delamination from the substrate.

Evaluation of cutting performance was done by turning test using WC-Co insert (ISO SNMA) PVD coated thick TiAlN and commercially available CVD coated TiCN/Al2O3 coatings were used as references. . In addition, two types of thick PVD TiAlN coatings were used: monolayer and bias multilayer. Regarding the cutting conditions, cutting speed is 300 m/min., feed is 0.25mm/rev., depth of cut is 2mm, and work-piece is ductile cast iron (AISI 80-55-06).

From the comparison of flank wear width, PVD coatings showed improved tool life by optimizing coating conditions. Especially, bias multilayered TiAlN achieved longer tool life than CVD coating: CVD coating almost reached the tool life (as defined 300 microns of flank wear) after 2700 m of cutting length, whereas flank wear width was much smaller for PVD coating. Crater wear depth of CVD coating reached 16mm after cutting of 2700 m, but PVD coating showed only 4 microns of crater wear, which proves good wear resistance.

Additional cutting tests result using different coating systems and discussion on the effect of coating properties such as hardness, oxidation resistance and residual stress of coating, and substrate material on wear mechanism will be presented.

Tungsten oxide (WO₃) is electrochromic under chemical insertion of metallic cations. This unique electro-optical property is due to the change of tungsten valance states from VI to V or IV to V. Such switch of multiple states enables tungsten oxide to be transparent or opaque according to the compositions and structures of W(IV), W(V) and W(VI). Since (WO₃) is mostly monoclinic in the temperature from 17 to 330 °C, it is very interesting to understand the underlying mechanism of valance-state change during the chemical insertions; as such electro-chemical changes are very useful for many practical applications, for instance, the vision shielding or detective sensors. In this study, the tungsten oxide thin film was prepared by DC reactive sputtering using physical vapor deposition. The chamber is monitored by the optical emission spectrometer and mass spectrometer for the plasma field. After deposition, the films’ thickness and average deposition rates are measured by surface profiler. The structure, elemental and chemical compositions of films are assessed by the XRD, EDS, XPS, Raman spectrometer, and ellipsometry. For the electrochromic measure, a UV-Vis-NIR spectroscopy is employed to quantify the optical transmission under different deposition conditions as well as after the chemical insertion process. A correlation between the control parameters of sputtering process and structures, properties of films shall be carefully examined to understand the forming mechanisms of films. This correlation provides a possible way of optimizing the sputtering conditions to fabricate appropriate tungsten oxide thin films for different applications.

Spotless arc Activated Deposition (SAD) combines electron beam high-rate evaporation using an axial electron beam gun and a spotless arc discharge burning in the metal vapor of a hot evaporating cathode. The SAD process is suitable for the evaporation of high-melting metals such as titanium, zirconium or tantalum, provides a high deposition rate of up to 2000 nm/s and enables a reactive mode of operation for the deposition of oxides, nitrides or other compounds with a deposition rate between 20 and 100 nm/s.

The limitation of the long-term stability of the SAD process that was caused by coatings deposited at the anode equipment could be overcome by introducing a dual crucible technology. Whereas evaporating metal in the first crucible acts as cathode, evaporating material in the second crucible forms the anode of the arc discharge. Both plasma electrodes evaporate and are in contact with vapor and reactive gas so that the plasma process is no longer disturbed by the coating of electrodes.

The challenging process conditions typical for plasma-activated high-rate electron beam evaporation have triggered substantial equipment innovations concerning the electron beam guns, the high-voltage power supplies and the control systems, too. As an enabling tool for advanced coating processes, a new class of high-power (up to 300 kW) electron beam modules was developed. They excel in improved dynamic pressure decoupling stages between electron beam gun and deposition chamber (coating pressure up 30 Pa), enhanced acceleration voltage (up to 80 kV), and availability of high-dynamic beam scanning systems (up to ± 45° x 10 kHz) as well as mid-frequency power supplies (with arc recovery time in the 1 to 10 ms range).

The main process parameters and discharge characteristics were studied for the evaporation of pure titanium and reactive processing in oxygen atmosphere in order to deposit titanium dioxide coatings on steel strip. Dense titanium dioxide layers were deposited with a high refractive index between 2.3 and 2.5, measured by ellipsometry . This relatively high refractive index allows to create coatings with strong color effects based on thin film interference. At elevated temperatures crystalline titanium dioxide thin films, especially layers with predominantly anatase phase, were deposited. These layers exhibit super-hydrophilic properties and photo-catalytic activities after exposition to ultraviolet light.

Physical vapour deposition (PVD) coatings can be readily applied to complex systems, they are very thin coatings, often around 5 microns in thickness. The aim of this work is to investigate the effect of coating architecture on corrosion behaviour of scratch resistant PVD coatings.

The coatings include single and multilayered systems. It was found that the thinner the multilayers the better the scratch resistance of the coating, however the coatings were seen to contain growth defects which expose the substrate to corrosive media and reduce the corrosion resistance of the coating. These defects decrease the corrosion resistance of the coatings. Growth defects typically occur during coating deposition, all coatings contain a degree of microporosity making them susceptible to corrosion however PVD coatings contain microporosity as well as growth defects, these growth defects can cause local loss of adhesion, higher friction, voids, act like stress raisers and pitting corrosion.

In order to improve the tribological properties of PVD coatings it is important to minimise the density of these defects, many attempts have been made to reduce these defects but they have yet to be fully eliminated so corrosion resistance is still limited by defects. These growth defects are non-uniformly distributed throughout the coating, their form, size and density depend on the deposition time, deposition techniques, deposition parameters, substrate position in the vacuum chamber, substrate orientation and rotation mode, and the surface conditions of the substrate

These growth defects are caused by various factors including substrate surface irregularities such as pits and asperities, foreign particles such as dust, debris or residues from grinding, blasting and polishing, or by the deposition process: depending on the deposition conditions microdroplets can form and fall onto the surface.

In order to identify which defects are most detrimental to coating performance, and which aspects of coating production are responsible for them, a set of samples with 4 different surface finishes were chosen: ground to 1-2 Ra, ground to 1200 grit, electropolished and microblasted.

In addition to the different surface finishes on the substrates, the effect of details of the deposition conditions was investigated. Coatings were deposited on the 1200 grit substrates under 4 different conditions: standard clean conditions, clean room conditions (including a thorough clean of the chamber prior to deposition). The effect of intermediate etching is also investigated.

Cathodic arc and compound cathodes is viewed as a convenient approach to generate plasmas with several ionic species for synthesis of multi-element films. However, there is generally a discrepancy between the cathode composition and the resulting plasma- and/or film composition. We present analysis of plasma chemistry and charge-state-resolved ion energy of dc arc plasma generated from Ti_1-xSi_x (0 ≤ x ≤ 0.25) Ti_1-yAl_y (0 ≤ y ≤ 1) and Ti_1-zC_z (0 ≤ z ≤ 0.25) cathodes, commonly used for synthesis of, e.g., hard and wear resistant coatings. In vacuum, the metal ion energies range up to > 100 eV, though this range, as well as the average ion energy, can be tuned by choice of cathode composition. Through related thin film synthesis and cathode surface analysis, the correlation between cathode-, plasma-, and film composition has been explored. There is a loss of primarily lighter elements (C, Si) in the film as compared to cathode composition. Our analysis indicates that this is likely due to a combination of processes at the cathode surface during plasma generation, and ion-surface interaction at the film during synthesis. Furthermore, for selected Ti-Si and Ti-Al cathodes, the plasma composition showed a lower Si/Al content compared to the cathode composition, yet concurrently deposited films were in accordance with the cathode stoichiometry. Hence, a significant contribution to film growth from neutrals is inferred. The macroparticle generation is also a function of cathode composition as well as process gas and pressure. Cathode surface analysis show correlated differences in phase formation and type of arc spots, and we therefore suggest macroparticle formation to depend on surface chemistry and cohesive energy of the phases present.

Compressor blades consisting of martensitic steel are exposed to foreign particles like sand, dust, ice particles and volcanic ash causing erosion. Depending on the angle of impact of the erosive particles the blade geometry changes due to damages at the edges. Such effects lead to the reduction of efficiency and increased fuel consumption. For protection of compressor blades, PVD coating systems are used. Nanocomposite (nc) coatings such as TiAlSiN are a promising candidate for the use as erosion resistant coating. TiAlSiN coatings consisting of hard (Ti, Al)N particles in an amorphous Si₃N₄ matrix are characterized by a high hardness up to 40 GPa or higher and plastic deformation resistance with increasing ratio of hardness H to elastic modulus E_IT. Typically, nc-TiAlSiN is deposited by means of different physical vapor deposition (PVD)-technologies. The low deposition rates limit the coating thickness to a few microns causing an early delamination of the coating under erosive particle bombardment. High Speed (HS)-PVD technology is a promising alternative for the deposition of nc-TiAlSiN coatings. The non line-of-sight characteristic allows the deposition of nc-TiAlSiN on compressor blades with a homogenous coating distribution. High gas flow rates ensure the deposition of coatings with a thickness up to 50 µm in less time compared to conventional PVD techniques.

In this study, TiAlSiN nc coatings were deposited by HS-PVD technology on martensitic steel X3CrNiMo13-4 (AISI F6NM) used in compressors. In this work, the investigations are focused on adhesion towards substrate and erosion resistance behavior. The morphology and the coating thickness were analyzed using scanning electron microscopy (SEM). The nc-TiAlSiN coating revealed a coating thickness of about 50 µm. Chemical composition of Ti, Al and Si was measured using X-ray spectroscopy (EDS). X-ray diffraction (XRD) measurement was used for phase analysis and proved the presence of crystalline AlN and TiN peaks. Investigations on the formation of the nanostructure were carried out by means of high resolution transmission electron microscopy (HR-TEM). The nanocrystalline (Ti,Al)N grains are embedded in an amorphous Si₃N₄ matrix. Hardness and elastic modulus were analyzed using nanoindentation method. Evaluation of the adhesion between substrate and nc-TiAlSiN coatings was made by scratchtest and impact testing. At the end nc-TiAlSiN coatings were tested regarding erosion resistance in a test rig for solid particle erosion. The angle of impact was varied between 0°-90°. The results revealed the ability of nc-TiAlSiN for an application as erosion resistant coating on compressor blades.

An industrial coating equipment, commercially used for the industrial-scale deposition of wear protection coatings on tools by means of vacuum arc evaporation with the rotating cylindrical cathodes technology, has been modified for reactive magnetron sputtering. We use a central cylindrical magnetron to achieve high-rate deposition of dense hard Al_xTi_1-xN, Al_xCr_1-xN, TiN and related coatings. With a D.C. power of 25 kWatt and one-fold rotation of the substrate tools, the resulting deposition rate is 10 µm/hr over a total area of 0.3 m². Thus the resultant high throwing power amounts to 3 µm∙m²/hr. Scanning electron microscopy studies were not able to resolve any columnar structure, they actually suggest a featureless, somewhat fibrous morphology of the coatings. Investigations with transmission electron microscopy at higher resolution revealed a fine columnar (diameter of about 25 nm) and extremely dense microstructure, with columns oriented in the direction of the film growth. Atom probe tomography studies perpendicular to the growth direction of the coatings proofed this extremely dense microstructure by exhibiting no change in chemical composition across the column boundaries over an investigated length of about 200 nm. The load-invariant hardness of the Al_0.57Ti_0.43N coatings reached a value of 33.4 ± 1.5 GPa and the elastic modulus amounted to 466 ± 15 GPa. Along with a high deposition rate, significantly higher than that achieved by vacuum arc and by conventional DC as well as by high power pulsed magnetron sputtering, the surface of the deposited coatings is very smooth with average roughness R_a= 0.06 µm and maximum roughness R_z= 0.72 µm. We present the design of the system and the method of the control of the operation of the magnetron in the transition regime between metallic- and partially- poisoned target mode that assures high deposition rates of stoichiometric nitrides. Several examples of cutting tests of different operations (milling, drilling) will be presented to show the excellent cutting performance of tools coated with this techniques as compared with conventional one. This technique is available in the commercial industrial coating unit π 411 of the company Platit.

Reactive Plasma Deposition (RPD) is a new method which enables to deposit various kinds of thin films without metal particles. Furthermore, RPD system, compared to conventional sputtering or cathodic arc ion plating (CAIP) technique, is expected to achieve highly ionized plasma. With the highly ionized plasma, the mechanical properties and wear resistance of the films deposited by RPD method are expected to be different from other processes, but few researches related to deposition of quasi-binary nitride films by RPD method are carried out.

In this paper, in order to investigate mechanical properties of cubic Al-Ti-N films deposited by RPD method, Al-Ti-N films were deposited onto cemented carbide substrates by using a dual source type RPD coating system. Ti and Al were evaporated respectively by plasma irradiation from pressure gradient plasma guns in nitrogen atmosphere. Deposition temperature was 420^oC and negative bias voltage of 100V was applied to the substrates. Evaporation rate from each source was measured by quartz crystal microbalance sensor and fixed to certain value by adjusting the gun power. By changing these rates, atomic ratio of Al / (Al+Ti), x, in the films was changed from 0.38 to 0.88. The films were characterized by X-ray diffraction (XRD), electron probe microanalyzer (EPMA), transmittance electron microscopy (TEM) and nano-indentation. The degree of ionization of the metal plasma was estimated by quartz crystal microbalance and biased grid mesh.

The film with x = 0.66 showed dual phase, mixture of NaCl type (B1) and Wurtzite (B4) structures, while the film with x = 0.44 and x = 0.88 showed B1 and B4 single phase. The films with x = 0.44 exhibited cubic phase and as high hardness as CAIP. The compressive stress of the B1 film was greater than 9 Gpa while the films by CAIP showed 5 Gpa. The films were oriented to (2 0 0) or (2 2 0) for B1 and (1 1 -2 0) for B4.

The relationship between compressive stress in the film and energetic factors of plasma, e.g. the degree of ionization of depositing atomic flux, is explained by C. A. Davis [1]. The model states that magnitude of the maximum stress of the films is dominated especially by the degree of ionization of atomic flux onto the substrate. The degrees of ionization γ in RPD were estimated to be approximately 0.7, and compressive stress of 9 Gpa obtained by RPD method presented good agreement to a calculated value from Davis’s equation with this γ. The Al-Ti-N film with high compressive stress presented longer tool life and better wear resistances than conventional Al-Ti-N films by CAIP in the continuous turning of alloy steel.

CrTaSiN coatings were prepared using reactive magnetron co-sputtering on silicon wafers and cobalt-cemented tungsten carbide substrates to evaluate its feasibility for protective purpose on glass molding dies. A Ta₂₂Si₁₉N₅₉ coating process was used as the basis to evaluate the effects of Cr addition in oxidation resistance and mechanical properties. The nitrogen flow ratio (N₂/(N₂+Ar)) was set at 0.4 to fabricate the CrTaSiN coatings with an over-stoichiometric ratio, N/(Cr+Ta+Si)>1, for a rock salt structure. The CrTaSiN coatings, with a Cr content of 1–11 at.%, exhibited a nanocrystalline or near amorphous phase, a nanohardness of 14.6–16.0 GPa, and a surface roughness of 0.4–1.0 nm. Annealing treatments were conducted in a 1%O₂–99%Ar atmosphere at 600 ° C for 500 min, an oxidation-accelerating condition, or a thermal cycling annealing at 270 and 600 °C under an atmosphere of 15 ppm O₂–N₂, a realistic glass molding atmosphere in mass production. The outward diffusion of Si resulted into the formation of an amorphous oxide scale, which maintained a surface roughness of 1 nm. The Cr-addition maintained the nanohardness at 19 GPa after 1500 cycles thermal annealing.