ICMCTF2019 Session F2-2-ThA: HiPIMS, Pulsed Plasmas and Energetic Deposition II

High power impulse magnetron sputtering (HiPIMS) has been shown to deposit W films with desirable physical characteristics including higher strength and uniformity than films deposited with traditional direct current magnetron sputtering (DCMS) under similar processing conditions. In-situ stress measurements paired with ex-situ film characterization and Langmuir probe plasma measurements were used to investigate the fundamental differences between DCMS and HiPIMS film deposition over a range of chamber pressures with and without applied substrate biasing.

LLNL-ABS-758580

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Support was provided by LDRD program 17-ERD-048 at LLNL.

Vanadium dioxide (VO₂) has attracted a great interest for smart coating applications because of its promising thermochromic properties. Its main characteristic is to transit from a semiconductive state to a metallic state at a phase transition temperature (Tc) of 341 K which is closer to room temperature (300 K) than any other thermochromic material. Nevertheless, the fabrication of high-quality VO₂ films by conventional direct current or radio-frequency Magnetron sputtering (DCMS or RFMS) requires an annealing step over 450°C which limits its compatibility with temperature-sensitive substrates. High power impulse magnetron sputtering (HiPIMS) is a novel sputtering method, developed recently for the deposition of VO₂ films. It is characterized by its high ionization degree of the sputtered species, which allows the control of film microstructure through ion bombardment. Due to this additional energy, the annealing temperature can be reduced for the development of functional VO₂. In this work, we show that HiPIMS supply permits to deposit dense stoichiometric crystalline VO₂ thin films at lower temperature.

Traditionally, growth of dense refractory thin films by means of dc magnetron sputtering employs gas-ion bombardment to ensure sufficient adatom mobility during deposition at lower temperatures T_s /T_m < 0.3 (T_s and T_m: growth and melting temperature in K). This approach often results in significant concentrations of trapped inert gas atoms with accompanying high film stresses leading to cohesive failure and film/substrate delamination. A possible remedy is offered by metal-ion-synchronized bias durings high-power pulsed magnetron sputtering (HIPIMS). The inherent separation of metal- and gas-ion fluxes that stems from gas rarefaction, provides the ability to separate, in both time and energy domains, metal ions and gas ions incident at the substrate. In this approach, a substrate bias potential is synchronized with the metal-ion-rich portion of the HIPIMS pulse. Metal-ions, as opposed to noble-gas ions, are primarily incorporated at lattice sites. This, together with dramatically-reduced concentrations of trapped gas ions, results in lower compressive stresses. Moreover, the metal-ion mass, incident flux, and impact energy can be independently controlled to optimize momentum transfer and provide the recoil density and energy necessary to eliminate film porosity at low deposition temperatures. This novel approach expands the PVD process envelope to allow the use of temperature-sensitive substrates as well as synthesis of supersaturated alloys.

Thin film metallic glass (TFMG) has been considered in the biomedical applications due to its better corrosion resistance, which attracts lots of attention from academic and industry. In this study, a co-deposition system consisting of a high power impulse magnetron sputtering (HiPIMS) and a radio frequency (RF) power supply was used to prepare Zr-Ti-Si and Fe-Zr-Nb TFMGs on the surface of 316 L stainless steel specimen.

The chemical composition, microstructure and surface roughness of Zr-Ti-Si, Fe-Zr-Nb TFMGs were analyzed by a FE-EPMA, FE-SEM and AFM respectively. The nanoindentation, scratch test and HRC-DB adhesion test were employed to evaluate the mechanical and adhesion properties of TFMGs. The corrosion resistance of TFMGs in 3.5 wt. % NaCl aqueous solution was conducted by a potentiodynamic polarization test. The MG-63 cell line (human osteosarcoma) was used to investigate the biocompatibility of coatings. Finally, the animal tests were executed to examine any allergy, poisoning or carcinogenic reaction was brought by the TFMGs to the rats.

Through the in-vitro cell test and in-vivo animal tests, both Zr-Ti-Si and Fe-Zr-Nb TFMGs exhibited better biocompatibility because of their good corrosion resistance, good hydrophilic properties and improved adhesion on the 316L surface. We can conclude that Zr-Ti-Si and Fe-Zr-Nb TFMGs have very excellent potential application in the biomedical field.

When synthesizing magnetron sputtered films with a complex stoichiometry, integrating the desired coating constituents in one target material is favorable to avoid a nanolaminar film depositions and to enable a homogenous film growth. In contrast to alloyed targets, segmented plug targets allow to merge elements with different physical properties in one target material. Two targets, amalgamating 20 and 48 hot-pressed 80Cr5Si15W (wt. %) plugs, respectively, into a monolithic aluminum target were fabricated and employed in a direct current magnetron sputtering process to deposit AlCrSiWN films on high-speed steel (AISI M3:2, 1.3344). Furthermore, an AlCrSiN film, which served as a reference, was deposited using Cr, Si, and AlCr24 targets. The cathode powers for the Al(CrSiW)20 and Al(CrSiW)48 targets were varied between 3 and 7 kW to analyze how differently composed targets and various cathode powers affect the microstructure and tribological properties of the sputtered films.

The results revealed that the chemical composition as well as the thickness of the films are strongly dependent on the target setup. For all AlCrSiWN films, the Cr and Al contents predominated (19‑29 at%), while the Si and W contents varied between 2‑3 at. %. This indicates that chromium is preferentially sputtered from the alloyed CrSiW plugs. The film, which was deposited by applying 7 kW to the Al(CrSiW)20 and 3 kW to the Al(CrSiW)48 targets had the highest hardness (27.66 ± 1.87 GPa due to a dense coating growth and a finer crystalline structure. All AlCrSiWN films had a preferred (111)‑(CrN/WN) orientation, which exhibited a finer crystalline growth with an increasing cathode power. Analyses of the tribological behavior of the AlCrSiWN films further revealed a significantly lower friction coefficient (≤ 0.4 ± 0.04*10^-5 mm³/Nm) when sliding against Al₂O₃ balls when compared to the AlCrSiN reference system.

Due to their properties, nanocomposite coatings offer outstanding advantages compared to single-phase coatings. However, the choice of materials to produce such nanocomposite coatings is severely limited. Basically, there are only three mechanisms that lead to the formation of nanocomposite structures. One possibility is the use of two metals which, in certain compositions, reveal a miscibility gap and form mixed crystals with different structures. Alternatively, metals with strongly different nitrogen affinities also form the desired nanocomposite structure. The third mechanism is based on the use of a nitride-forming transition metal, which is nanocrystalline embedded in an amorphous matrix.

To allow an unrestricted combination of materials, the approach in this project is a decentralized production of nanoparticles and the thin film, whereby the films growth of both phases is combined on the substrate material. For this attempt, an atmospheric pressured arc reactor is used to generate the TiN nanoparticles with mean primary particle sizes of 7 nm to 20 nm agglomerated to clusters of about 120 nm. To inject these nanoparticles in-situ into the vacuum PVD process, an aerodynamic lens is utilized. The purpose of the aerodynamic lens is to separate the process gas from the nanoparticles to only introduce the nanoparticles into the PVD process. Thus, depending on the coating materials used, nanostructured thin films of type nc-MeN / nc-MeN or nc-MeN / nc-Me can be produced in addition to already existing systems, such as nc-MeN/a-nitride and nc-MeC/a-C, synthesized by conventional PVD processes. Within this context, the metal (Me) can also be nitrogen-affine in the new approach.

In this project, TiN nanoparticles are employed and embedded into magnetron sputtered CrN thin films. Initially, the influence of selected PVD deposition parameters, such as the temperature, chamber pressure, and bias voltage, is examined on the morphology and crystallite size of the TiN nanoparticles. Additionally, the influence on the particle distribution on the substrate is discussed. The nanoparticles were successfully deposited on silicon wafer and it was found out that the particle distribution is affected more by the deposition parameters than the particle properties mentioned. This enables the production of nanostructured PVD thin films by means of external nanoparticle injection without the particle properties being subsequently influenced by the deposition parameters during the coating process.