ICMCTF2014 Session B4-2: Properties and Characterization of Hard Coatings and Surfaces

Under typical sputter-deposition conditions, 25-30% of the interfaces in Fe/VC multilayer films have the (100) orientation. The lattice constants of Fe and VC are such that these interfaces are coherent with a lattice mismatch of about 2.7%. It is believed that such coherent interfaces will affect the hardness and fracture toughness of these films. Fe/VC multilayer films with a fixed bilayer period (Λ = 8.8nm) and variable modulation Fe fraction (Λ _Fe/Λ ranging from 0.6 to 0.9) were deposited via dc magnetron sputtering. X-Ray diffraction and transmission electron microscopy were used to investigate the nanoscale layer structure of these films. Hardness was determined by nanoindentation and fracture toughness by microindentation techniques. Throughout the entire modulation Fe fraction investigated in this study, the hardness was enhanced over the rule-of-mixture value. Even at Fe fraction of 0.9, the hardness value was 15 GPa, enhanced by about 80% over the rule-of-mixture value of 9 GPa. If fracture toughness were to scale inversely as hardness, this film should have a fracture toughness of about 1.5 MPa-m^1/2, compared with our measured value of 3.4 MPa·m^1/2. We propose that coherent Fe/VC interfaces play an important role in the observed hardness and toughness enhancement.

Keywords: Hardness, Fracture Toughness, Multilayers, Magnetron Sputtering, Nanoindentation

This is done using a patented method to separately actively heat and control the temperatures of indenter and sample resulting in minimal/no thermal drift during the high temperature indentation. The instrumentation allows reliable nanomechanical testing (e.g. nanoindentation, nano-scratch, micro-pillar compression, micro-cantilever bending) to 750 degrees C and above. To achieve higher temperatures without indenter or sample oxidation an ultra-low drift high temperature vacuum nanoindentation system capable of testing to 1000 degrees C has been developed.

High temperature nanoindentation data for a wide range of nitride-based hard coatings on cemented carbide have been used to develop design rules for coating optimisation for different machining applications. The coatings studied show large differences in how their hardness, modulus and H/E vary with increasing temperature. The interrelationship between the high temperature mechanical properties and the coating system’s adaptive behaviour and tribo-film formation and ultimate performance is investigated. Overall, the high temperature nanoindentation data show excellent correlation to coating life under severe high speed machining applications.

Cathodic arc evaporated Ti_1-xAl_xN is commonly used as a hard coating on metal cutting inserts. It is well known that the as-deposited unstable cubic (B1) state of Ti_1-xAl_xN decomposes in two steps at elevated temperatures. The first, beneficial step, where coherent nanostructured cubic c-TiN and c-AlN rich domains are formed during spinodal decomposition, is followed by a detrimental transformation of c-AlN into its stable hexagonal form (h-AlN, B4).

In the present work, we have studied the decomposition of arc evaporated Ti_0.50Al_0.50N and Ti_0.33Al_0.67N during heat treatment in vacuum by in-situ synchrotron x-ray diffraction. Three isothermal temperatures (between 950 ºC and 1100 ºC) per sample have been used with a time resolution of approximately three measurements per minute. The measurements were conducted on powder of Ti_1-xAl_xN which makes a quantitative analysis of the diffractograms possible. In addition, in-situ small angle x-ray scattering measurements were conducted to explore details of the wavelength evolution of the spinodal decomposition, thus providing information about the critical size of the c-AlN rich domains prior to the onset of the h-AlN transformation.

The results provide valuable information of the fractional cubic to hexagonal transformation of AlN in Ti_1-xAl_xN as a function of time and yield an activation energy between 3.1 and 3.6 eV/at. The onset of the hexagonal transformation occurs at about 50 ºC lower temperature in Ti_0.33Al_0.67N compared to Ti_0.50Al_0.50N. A critical wavelength of the cubic domains of about 8 nm was observed for Ti_0.33Al_0.67N and about 12 nm for Ti_0.50Al_0.50N. Furthermore, the conversion rate from cubic to hexagonal AlN in Ti_1-xAl_xN was significantly lower for the low Al content Ti_1-xAl_xN powder. For example, Ti_0.33Al_0.67N has completed the transformation to h-AlN after 120 min at 1000 ºC while Ti_0.50Al_0.50N has only completed 40% during the same annealing time.

The grain size, residual stress and strain set some of the properties of the coatings made by thermal spray, this is due to the process and hi cooling rates, which trend to produce nano-grains with hi residual stress and strains, or even amorphous metals.

This research use the thermal spray wire arc in order to make steel coatings using the alloy Fe-W-Cr-Nb and traditional steels AISI/SAE 1020 and 420 as follows: Single layer coatings of each material and simultaneous coatings of two materials at the same time using the alloy and one of the traditional steels. That in order to create pseudo alloys. The grain size, residual stress and strain were computed using mathematical approximations of the X-ray diffraction results and transmission electron microscopy (TEM). The coatings also are characterized by using, optical microscopy, scanning electron microscopy (SEM) and micro-hardness.

It was found that the single layer coatings of the alloy had the smallest grain size, (nano grain size) also those kind of coatings have the biggest residual stress and therefore the biggest strains, this is possible due to the powder inside the original metal core can be nano-grain nucleant, increasing the nucleation rates, if that is accompanied with the cooling rates own of the process 10⁷ K/s can be obtained nano structures, as well as were observed by TEM. On the other hand the traditional steel coatings have typical grain size found in the literature, also these kind of coatings have smaller residual stress and strain than the alloysingle coatings. Finally using the alloy and the traditional steels, the pseudo alloys obtained had residual stress and strain smaller than its precursors, conformably grain size were bigger than the traditional steels.

This effect may be due to changes at the direction, heat rate, thermal conductivity thermal expansion among others, details of the computation and correlation with the results are exposing at this research presentation.

Hydrogen cracking of high-strength steels is a major concern in steel processing and service, and occurs in several applications, such as cracking of rolled steel products, cold cracking of welds, and as a result of corrosion in H₂S environments.

Low-permeation hard coatings can be used as wear-resistant hydrogen permeation barriers. When coated on stainless steel they can reduce the rate of hydrogen transport. And they might be useful for sterling engines, tritium containment, or components of hydrogen fuel cells.

The hydrogen permeation behavior of BN-coated SUS316L stainless steel was investigated. The c-BN (cubic boron nitride) coating, deposited by magnetically enhanced plasma ion plating, was more effective to reduce the rate of hydrogen permeation through stainless steel than TiN coating. The c-BN coating can be used for high-temperature and wear-resistant applications as hydrogen permeation barriers.

Multilayer films have attracted great attention in recent years because of their promising properties. Comparing to monolayer films, multilayer films usually demonstrate increased hardness and wear resistance. TiN film has been widely used in industry due to its high hardness, good wear resistance and anti-corrosion performance. However, due to the relatively large residual stress and weak fracture toughness, TiN film cannot meet the more stringent requirements in many cases. So, TiN/metal multilayer films such as TiN/Ti, TiN/Cu, TiN/W, and etc., have been extensively studied.

Three commercial types electroless nickel phosphorous baths – low, mid and high phosphorous – and 2 lab-developed electroless nickel-boron bath – both based on borohydride but differing by their stabilizer, lead and thallium salts respectively – were used in this study. Samples were investigated in the as-deposited and heat treated condition (after treatment at 400°C for 1 hour).

The samples were fully characterized by GD-OES, SEM and x-ray diffraction in order to assess their chemistry, morphology and structure. Their hardness was assessed by Knoop microindentation and the boron-stabilized bath presented the better behavior. The same coating also presented the better properties during Taber abrasion test with CS-10 abrasive wheels. Some coatings were also submitted to pin on disc wear test. Scratch test behavior of the various samples was also investigated with the aim of assessing the adhesion of the various coatings on the substrates and identifying the damage mechanisms of the various electroless nickel deposits.

In this paper, the mechanical, tribological and corrosion properties of two hydrid coating systems were evaluated. The first coating system consists of a tungsten-tungsten carbide W(WC) top layer and a laser cladded stellite interlayer deposited onto 316 stainless steel substrate, and the second one consists of the same W(WC) top layer and a HVOF sprayed and fused colmonoy C88 interlayer onto inconel 718 substrate. X-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy were used to analyse the microstructure of the coating layers. Micro-indentation technique was utilised to measure the surface hardness as well as the hardness profile of the coating systems. Rockwell indentation technique was used to evaluate the adhesion of the coating as per CEN/TS 1071-8. Tribological properties were evaluated by pin-on-disk measurements and corrosion resistance was measured by potentiodynamic polarization and electrochemical impedance spectroscopy EIS.

Hardness profile measurements (on cross section) showed that W(WC) and Stellite have hardness values of 14 GPa and 7 GPa respectively while Colmonoy hardness varies between 7 GPa and 13 GPa depending on the phase under the indenter. On the other hand, W(WC) surface hardness was shown to depend strongly on the indentation load. Furthermore, W(WC) layer exhibits excellent adhesion on both stainless steel and inconel substrates with and without the use of interlayers, however the tribological measurements have shown that W(WC) wear resistance was improved by almost 30% with the use of hard interlayer. In addition, W(WC) layer showed excellent corrosion protection as no pitting was observed after potentiodynamic polarization testing.