ICMCTF2011 Session B6-2: Application-Oriented Coating Design and Architectures

The mechanical, optical and electrical (resistivity, dielectric constant) macroscopic properties of nanocomposite films strongly depend on chemical composition and nanostructure. Different types of self-assembled composite material such as: ZrN-SiN, AlN-YN, MoSe-C and BaTiO₃-CeO₂ have been investigated. In such nanocomposite thin films the crystallite size is on the order of a few nanometers and hence the grain surfaces and boundaries have an important effect on the physical properties. The arrangement and the chemical composition of the phases must therefore be known precisely.

The limits of the standard characterization techniques in revealing such composite nanostructures will be discussed in order to emphasize the need for physical models. The limits to experimentally confirm such models motivate us to employ unconventional investigation techniques such as electrical and piezoelectric measurements or specialized HRTEM image processing. The continuity of insulating SiNx-layer on conducting ZrN-crystallites or the continuity of BaTiO₃-phase from bottom to top electrode were evidenced by electrical measurements.

Hard nanocrystalline coatings exhibit depth-gradients of crystallographic texture, strain, lattice defects, composition, grain size and morphology. This is a fundamental aspect determining the mechanical and thermal properties of the coatings. Up to now, mostly volume-averaged structural parameters of coatings were determined using X-ray diffraction studies in reflection geometry. The aim of this contribution is to introduce a new X-ray diffraction approach to characterize depth gradients of nanostructure and strain at the cross-section of hard nanocrystalline coatings. The new technique was developed at ID13 beamline of ESRF (Grenoble, France). The approach is based on position-resolved wide-angle X-ray diffraction performed in transmission geometry with a monochromatic beam of 100 nm. The new technique opens the possibility to map the structural properties of the coatings on the nano-scale. In the combination with finite-element modelling, the approach allows to assess the residual stress gradients across compositionally graded coatings. Finally this new approach opens a unique opportunity to correlate coating performance and actual nano-structural design.

The main objective of the authors is the development of an surface design tool in terms of an expert system, which integrates analytical models of all knowledge steps (from surface processing to material structure in all scales to generic material properties to tribological performance) of the surface design process, in order to enable companies to quickly and reliably design or optimize application-tailored coatings and, therefore, drastically reduce trial-and-error testing and complicated modeling. Meaning, that one specifies the intended application as well as the desired lifetime of a surface and the surface design tool automatically derives, firstly, the necessary tribological performance, the mechanical material properties, the material nano-/micro-/macro-structure, and finally an appropriate processing technology and corresponding processing parameters based on the deposition equipment being at one's disposal. Scale-invariant analytical models for complex contact & load situations with arbitrary structured surfaces as well as wear prediction developed earlier [1,2] will be incorporated in order to predict wear and evaluate contact & load situations of the intended application during lifetime. Thus, it will be able to derive requirements concerning the surface with respect to its tribological performance, generic mechanical properties, and macro structure. However, there are no general analytical models for surface processing which enable one to predict the surface structure in all scales and resulting generic mechanical properties. Hence, the development of such a model is one sub-objective of the authors. In this work, the present development progress of this analytical model for the deposition process will be presented.

References

[1] N. Schwarzer: “Coating Design due to Analytical Modelling of Mechanical Contact Problems on Multilayer Systems”, Surface and Coatings Technology 133 –134 (2000) 397 - 402

[2] OptiWear: tribology experiment simulation software, http://www.siomec.de/OptiWear

A solid carbide, through coolant drill, specifically designed for drilling Ti and Ti alloys and produced without any PVD coating by the OEM, was used in a series of benchmarking studies by a number of collaborating companies. The drill performed well and as expected when used as per the OEM parameters.

However, by applying a state-of-the-art PVD coating and modifying the resulting surface, a significant improvement in productivity of over 45% was demonstrated, as well as a reduction in the measured wear, compared to the untreated drill ran at the standard conditions. This demonstration study also highlighted the potential to increase the productivity and increase lifetime without modification of the uncoated drill.

The commercial coating was produced in an Oerlikon Balzers INNOVA PVD system. Post treatment was based on NAF brushing. The effect of the coating on the finish of the drill, the subsequent post treatment and the progression of the wear were observed and measured using a novel light microscope and a traditional toolmaker’s microscope. Quality was determined by measuring the hole diameter, the hole finish and the burr on break through on test pieces cut from the test material. Torque and thrust measurements were carried out on the coated and uncoated drills.

Drill test data is affected by a large number of variables outside the control of a cutting tool manufacturer including different CNC machines, tool holding chucks, work piece holders and coolants. This can influence the determination of optimal machining parameters and therefore material removal rate in a particular application, consequently recommended speeds and feeds are by necessity conservative. At the same time drill manufacturers and end users employ benchmarking techniques, like a drill test, to compare different cutting tools. However, this can be problematic when potentially non-optimal conditions are used for some of the tools in the study. There is therefore a need for a drill testing methodology which is, on the one hand, able to discriminate the effects of tool substrate, surface finish, macro and micro geometry and tribological interactions and on the other, provide meaningful comparative information, given that different design parameters may influence measured outcomes. The main specifiable parameters that influence the forces involved in machining a particular material are the cutting speed, feed rate and depth of cut. This work explores the variability of the cutting forces during drilling in order to assist the development of a drill test by varying a number of controllable design and process parameters. The tools selected for the experimental work were ¼ inch M2 HSS jobber drills. The work piece material variables were AISI D2 and AISI P20 equivalents. Analysis of torque and thrust data while varying the cutting parameters was completed for a range of different tribological conditions in order to compare drills with modified coatings and finishes. The effects of pre and post treatment on the surface finish of the coated tools were quantified using an infinite focus microscope.

In modern power generation gas turbines water fogging, i.e. the injection of water mist into the turbine is used to augment the performance and efficiency by reducing the air temperature and increasing the compression ratio. The water droplets hit the leading edge of the first stage compressor with app. the speed of sound and can cause liquid droplet erosion (LDE) of the base metal. This leading edge erosion can lead to a reduction of the cord width over extended periods of operation. With some blade designs, this can lead to fatigue cracks which shorten the life of the blades. Various abatement solutions are presently under investigation: redesign of the airfoil shape to allow more material removal, laser peening to reduce the impact on fatigue life and protective coatings. We have investigated the effectiveness of substoichiometric TiN multilayer coating systems to protect the leading edge of the compressor blades against liquid droplet erosion. A full set of 1st stage compressor blades has been coated with two different Substoichiometric TiN multilayer architectures. After 10,000 hrs of field service the erosion damage progression in both coatings was evaluated. In a parallel effort an accelerated water jet test procedure has been developed to simulate the LDE process. A semi-quantitative correlation between the damage progression in the 10,000 hr field test and the water jet test confirmed the effectiveness of the water jet test as a meaningful tool to predict the coating performance in an LDE environment. The accelerated water jet testing is capable of simulating the LDE damage progression. The better performing coating of the two types investigated has a predicted life in excess of 50,000 hrs for the current environment.