ICMCTF2012 Session B5-1: Hard and Multifunctional Nano-Structured Coatings

Self-Organized nano-Labyrinth Structure in Magnetron Sputtered Zr_0.6Al_0.4N(001) Thin Films on MgO(001)

N. Ghafoor¹, L. J. S. Johnson¹, D. O. Klenov², B. Alling¹, I. Petrov^{1, 3}, J. E. Greene^{1, 3},

L. Hultman¹, M. Odén¹

¹Department of Physics, Chemistry, and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden

²FEI Company, Building AAE, Achtseweg Noord 5, Eindhoven, The Netherlands

³ [http://www.mrl.uiuc.edu/], [http://www.uiuc.edu/],104 S. Goodwin Avenue, [http://www.city.urbana.il.us/Urbana/], IL 61801, USA

A unique self-organized nanostructure is reported for (Zr_0.64Al_0.36)N thin films that forms during reactive magnetron sputter deposition from elemental targets on MgO(001) substrates. HRTEM and EDX/STEM imaging shows that it consists of a nanolabyrinth of alternating ZrN-rich and AlN-rich lamellae along the MgO<110> in plane directions, with typical width of a 4 nm and that extends throughout the film thickness. According to ab-initio calculations, a significantly higher solid solution mixing enthalpy of c-Zr_0.6Al_0.4N (∆H=0.392 eV/f.u), compared to c-Ti_0.6Al_0.4N (∆H 0.178 eV/f.u) results in a comparatively higher driving force for isostructural decomposition in ZrAlN during secondary phase transformation on post annealing. Growth kinetics and pseudomorphic epitaxial forces also effects the segregation to occur during growth.

Economic and technological progress as well as environmental concerns requires that modern equipment be designed with ever more stringent performance criteria, frequently pushing components to the very limits of their capabilities. Consequently, tribological deficiencies, such as lubrication breakdown, excessive wear and tribo-corrosion, are amplified leading to unnecessary operational costs, decreased efficiency and premature failure. Therefore, appropriate material’s selection for a given application must be guided by an accurate understanding of the intervening tribological processes.

Solid Particle Erosion (SPE) occurs in situations where hard solid particles present in the environment are entrained in a fluid stream, and impact component surfaces. It is well known that ductile materials erode predominantly by plastic cutting or ploughing of the surface, while brittle materials do so by dissipating the particle kinetic energy through crack nucleation and propagation. Although the most widely accepted brittle erosion models were developed more than 30 years ago, little has been published on how the material removal process differs for hard brittle coatings when compared to bulk materials. In this presentation, we outline the work performed at École Polytechnique on understanding the material loss mechanisms occurring during the SPE of hard coatings.

In the first part, we will discuss the validity of existing brittle SPE models when applied to the erosion of hard coatings. We examine the mechanisms by which surfaces dissipate the kinetic energy of impacting particles, and compare the erosive response of brittle bulk materials to that of hard coatings. Also, we investigate the means by which surface engineering can enhance erosion resistance, and correlate surface mechanical properties to the erosion behaviour of brittle bulk materials and coatings. We will show that the experimental results are also well supported by the finite element modelling of single particle impacts of coated surfaces.

Transition metal nitrides, such as CrN are highly attractive materials for a wide range of applications due to their outstanding properties like high hardness, excellent corrosion and oxidation resistance. Consequently, many research activities deal with their synthesis-structure-properties-relations. However, also because the fracture toughness of thin films is a difficult-to-obtain material property, only limited information is available on this topic. Therefore, this work is devoted to the study of the fracture mechanisms of CrN based thin films with the aid of in-situ scanning electron microscopy microbending, microcompression and microtension tests. The small test-specimens are prepared by focused ion beam milling of individual free-standing thin films. As generally monolithic coatings with their columnar structure provide low resistance against crack formation and propagation we perform our studies for CrN thin films and CrN/AlN multilayers. The latter offer additional interfaces perpendicular to the major crack-propagation-direction having different elastic constants and shear modulus and binding characteristics. Adjusting the AlN layer-thicknesses to ~3 and ~10 nm allows studying the impact of a cubic stabilized AlN layer and an AlN layer composed of cubic, amorphous and hexagonal fractions being extremely sensitive to stress fields.

Cr-based nanocomposite coatings are attracting increasing attention for use as protective coatings in dry machining and aerospace applications where the components are consistently exposed to high temperatures. In this report, CrAlSiN nanocomposite coatings have been deposited with different silicon contents and at different substrate bias voltages using a lateral rotating cathode arc technique. Their composition, microstructure and mechanical properties were characterized using EDS, XRD and nanoindentation respectively and compared with CrN and CrAlN coatings deposited under the same conditions. The flow behaviour around the indent has been studied using AFM and TEM. The mechanical behaviour of the coatings has been determined both from room temperature tests after annealing at elevated temperatures and hot stage nanoindentation, allowing the hardness to be measured at temperatures up to 600 °C. The evolution of the structure and internal stress has been measured in both cases allowing the two approaches to be compared.

Hard Zr-B-C-(N) films were deposited on Si(100) substrates by pulsed magnetron co-sputtering of a single B₄C-Zr target (127 x 254 mm²) in various nitrogen-argon gas mixtures. The target was formed by a B₄C plate overlapped by Zr stripes which covered 15 or 45 % of the target erosion area. The N₂ fractions in the gas mixture were in the range from 0 to 50 % at the total pressure of the gas mixture of 0.5 Pa. The planar rectangular unbalanced magnetron was driven by a pulsed DC power supply (Rübig 120 MP) operating at the repetition frequency of 10 kHz and the average target power of 500 W in a period with a fixed 85% duty cycle. The substrates were at a floating potential and were heated to 450˚C. The target-to-substrate distance was 100 mm. The elemental composition of the films was determined by Rutherford backscattering spectrometry. X-ray diffraction measurements of as-deposited samples were carried out using a PANalytical X’Pert PRO diffractometer. Hardness, reduced Young’s modulus and elastic recovery were determined by a Fischerscope H-100B ultramicroindenter. Electrical resistivity was measured by four-point method. Hard (37 GPa) nanocrystalline Zr-B-C films with very low compressive stress (0.4 GPa) and high electrical conductivity (resistivity of 2.3×10^-6 Ωm) were deposited in argon discharge at the 15 % Zr fraction in the target erosion area. Hard (37 GPa) nanocomposite Zr-B-C-N films with low compressive stress (0.6 GPa) and even higher electrical conductivity (resistivity of 1.7×10^-6 Ωm) were deposited at the 45 % Zr fraction in the target erosion area and 5 % N₂ fraction in the gas mixture.

Many engineering materials require a combination of properties: wear-, corrosion-, and oxidation-resistance, thermal stability, high fatigue strength, and low friction coefficient. These properties can be achieved in hard coatings based on borides or boronitrides of transition metals. In the present work, Mo-B-(N), Cr-B-(N), and Ti-B-(N) coatings were deposited by magnetron sputtering of MoB, CrB₂, and TiB_x (x=1 and 2) targets in an Ar atmosphere or reactively in a gaseous mixture of Ar+15%N₂. To evaluate oxidation resistance, the coatings were annealed in the range of 800-1200⁰C in air. The structure of coatings was studied by means of X-ray diffraction, transmission and scanning electron microscopy, Raman and X-ray photoelectron spectroscopy, and glow discharge optical emission spectroscopy. The mechanical properties of the coatings were measured using nanoindentation and scratch-testing. The tribological properties were evaluated in air using both conventional and high-temperature ball-on-disc tribometer. The electrochemical tests were performed in 5N H₂SO₄ medium. The milling, drilling, and turning tests of the coated WC-Co and HSS tools were also performed.

The results obtained show that the coatings with optimal structure and elemental composition had hardness up to 55 GPa, elastic recovery up to 70%, friction coefficient against WC-Co ball below 0.5 and wear rate <10^-5 mm³N^-1m^-1. The tribological properties of coatings were shown to be stable in the temperature range of 20-500⁰C. The reactively deposited coatings showed the better corrosion and oxidation resistance than that of the nitrogen-free coatings. The lifetime of cutting tools coated with Ti-B-(N) and Cr-B-(N) coatings was increased by 1.5-2.5 times compared with the TiN coating.