ICMCTF1998 Session E4-F1-2: Mechanical Characterization - Micromechanical Testing & Modeling (2)

Enhanced tribological properties have been observed after treatment with pulsed high power ion beams, which results in rapid melting and resolidification. We have treated and tested 440C and 15-5 steel. Ti and Al samples were also treated after sputter coating to produce surface alloying. The samples were treated at the RHEPP-1 facility at Sandia National Laboratories (0.5 MV, 0.5-1 μs at sample location, <10 J/cm², 1-5 μm ion range). The tribological properties of the samples were determined using a linear reciprocating tribometer with ball-on-flat geometry. Changes in the treated layers were characterized by nanoindentation hardness testing, X-ray diffraction, microprobe cross-section analysis, SEM, and topographical analysis. The thickness of the treated layers ranges from 2-5 microns. Concentration profiles of the surface alloyed Ti samples were determined by RBS.

Increases in hardness and durability were found after treatment. We have observed a reduction in size of second phase particles and other microstructural changes in 440C. The hardness of treated 440C increases with ion beam fluence and varies with the ion species used for treatment. Ion beam treatment of Ti-Pt and Ti-Nb co-sputtered overlayers on Ti substrates increases the durability of these surfaces over untreated Ti and treated bare Ti. The surface alloyed layers show improvements in hardness up to a factor of 3 over untreated Ti. Al was coated with a Al-Si co-sputtered overlayer and treated, resulting in a hardness increase of a factor of two over untreated Al, along with increased durability. The magnitude of the changes in surface topography and roughness depend on the ion beam species used in the treatment.

This work was supported by the United States Department of Energy under Contract DE-AC04-94AL85000. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy.

Thin, hard ceramic layers are widely applied as tool coatings to increase resistance to wear and corrosion. An important property characterising the quality of coatings for these applications is their strain-to-failure, at which cracks appear in the coating. We have used pure bending to apply a large controlled strain to the coatings, while monitoring the formation and propagation of cracks in the coating. Experiments were performed using two different methods of failure detection.

The first method uses an electrochemical signal to determine the strain-to-failure of non-conducting coatings. During the experiment a constant electric potential is applied to the thin-film-and-substrate system, with the thin-film in contact with an electrolyte. The current of corrosion is measured, and is seen to change dramatically when the coating cracks and the electrolyte contacts the substrate (in our case stainless steel). This is a quick, easily performed and well reproducing method, and therefore suitable as a quality control instrument. To investigate if it is possible to extract more information from such measurements, eg. on the exact moment of crack initiation, crack density and evolution, we have performed additional similar experiments using in-situ SEM.

In these experiments identical pure bending conditions are applied inside a SEM allowing to obtain visual information on the same phenomena. The initiation and propagation of cracks, and the crack density can be studied and registered in this way, and the electrochemical curve can now be interpreted in these terms.

Numerous coatings of titaniumnitride (TiN) and titaniumcarbide (TiC) produced with PVD (unbalanced magnetron sputtering) have been investigated in this way. In this paper the relation between the electrochemical method and the in-situ SEM visual observations is presented and discussed.

The formulation and verification of accurate and sufficiently general hardness models applicable both to soft-on-hard and hard-on-soft composite systems, which may exhibit either fracture or plasticity-dominated coating responses, is vital for improved understanding of mechanical properties of coated systems, as well as for practical design aspects of surface engineering for mechanical applications. A recently proposed energy-based model for hardness-load dependence is applied here to a series of samples of electroplated Ni on Cu substrates, in which the hardnesses of both the coating and the substrate were controlled by bath formulation, heat treatment, and annealing.

It was found that the model accurately describes the transition between the coating-only hardness response at very low loads, and at penetration depths below approximately one tenth the coating thickness, to substrate controlled response at larger loads, and depths comparable or greater than the coating thickness. When once the coating-only hardness is measured or estimated, the model contains a single non-dimensional 'composite response' parameter, which depends on certain combination of system properties. The series of microhardness measurements discussed in the paper is used in conjunction with least square fits to the model predictions. One of the purposes of this analysis is to clarify the physical meaning of the composite response parameter. The implications of the results are discussed in view of the possible engineering and design applications of this approach.