A.B. Vannes

Surface modification and tribological behaviour of titanium and titanium alloys after YAG-laser treatments

Commercially pure Ti and Ti alloys substrates have been treated using a YAG-Nd laser radiation (1.06 μm). A large range of laser parameters have been tested resulting in very different coloured layers. X-rays diffraction and Raman spectroscopy have been performed in order to characterize the composition and crystalline structure of the coatings. Their roughness and thickness were also measured. Their tribological properties have been evaluated under fretting conditions against a 52 100 steel cylinder and compared with the fretting behaviour of uncoated samples. The evolution of composition and structure of the coating during friction has been studied.

Erosion by solid particles of W/W-N multilayer coatings obtained by PVD process

Abstract Sand-particle erosion remains a crucial problem in aeronautical engines, especially on compressor blades. These pieces are made from a titanium-based alloy, which is known to have a poor erosion resistance. To increase the endurance of these blades, it is planned to protect them with tungsten-based multilayer coatings obtained by PVD process. Coatings were first sputtered on flat samples, and their erosion resistance was evaluated using a horizontal projection device. The influence of several deposition parameters (Kr or Ar pressure, N2 flow) on erosion resistance was studied on two types of monolayer films: pure tungsten layers and layers reinforced with N atoms (called W–N). Based on these results, multilayer W/W–N coatings were then produced and tested. The observation of degradation mechanisms led us to study a new type of films, with a gradual change in N content. These gradient films showed an erosion resistance far higher than that of all multilayer coatings previously tested.

Adhesion and residual stresses determination of thermally sprayed molybdenum on steel

Abstract Thermally sprayed molybdenum coatings are widely used to combat degradation of components and structures due to mechanical wear. However, the behavior and durability of these coatings are extremely dependent on their properties and on the spraying conditions. To date, efforts forwarded to develop thermal spray coatings technology have been focused on increasing the durability and integrity of the coating. Improvement of adherence of the deposit to the substrate, minimizing residual stress level and reducing porosity are the main objectives of these works. The aim of this paper is to measure the adherence of flame-sprayed molybdenum on steel substrate and to determine internal stress distribution at the interface of the obtained structure. The influence of a nickel–aluminum (80%, 20%) bond-coat and/or a post-annealing at 850 °C for 1 h in vacuum on the adherence and the residual stresses is also studied.

Residual stresses and fretting fatigue

Controlled residual stress field, induced by conventional and ultrasonic shot peening in some reference materials has been measured using X-ray diffraction. Its influence on the tribological behavior of these treated samples has been studied and its evolution during the fretting tests has been followed and related to the observed wear mechanisms.

Study of sand particle erosion of magnetron sputtered multilayer coatings

Sand particle erosion remains a crucial problem in aeronautical engines. Blades are made out of titanium alloy known for its poor erosion resistance. To protect these pieces, tungsten-based multilayer coatings deposited by magnetron sputtering were developed. They were tested on a specific erosion rig. Tests allowed to choose deposition parameters leading to extremely resistant films. Impact tests were performed on several monolayer coatings to determine their mechanical properties in order to simulate erosion by finite element method.

Influence of an intermediate layer on the residual stress field in a laser clad

Abstract Large residual stresses are generally produced during laser surface treatments owing to their associated high thermal gradients and cooling rates. In laser clads the high tensile residual stresses usually obtained can produce cracking or limit the practical use of these deposits. To investigate the potential of intermediate layers in controlling the residual stresses, multilayer clads were produced on a high chromium martensitic steel, with an austenitic steel as an intermediate layer and stellite 6, a cobalt-based alloy, as the surfacing material. The stress profiles were calculated using the measurements of the deformation resulting from the successive electrochemical removal of thin layers. It was observed that the sign of the residual stresses in the surface alloy was not changed by one or more intermediate layers.

Evaluation of the thermal field developed during pulsed laser treatments: semi analytical calculation

Abstract A semi-analytical model has been developed and computed in order to evaluate the thermal field created at a sample surface during a pulsed laser treatment. At low computing costs, this model allows us to take into account the different evolutions of the physical parameters (absorptivity, thermal diffusivity and conductivity, etc.). Main equations are given and the model has been applied to estimate the maximum temperatures reached during a Nd–YAG (Q-switched) laser treatment of titanium samples. Results are compared with the experimental observations and further developments of our computing program are evoked.