ICMCTF2010 Session E4/G4: Coatings for Machining Advanced Materials and Advanced Manufacturing Methods

High speed machining of materials inherently generates large cutting temperatures, which not only reduce tool life but also impair the product quality. Though cutting fluids are widely employed to carry away the heat in metal cutting, they cannot be recommended in the light of ecological and economic manufacture. Hence, there arises a need to identify eco-friendly and user-friendly alternatives to conventional cutting fluids. Modern tribology has facilitated the use of solid lubricants. So, in recent years researchers have started machining with the use of solid lubricants with the aim of improving machining performance and overcome the limitations that arise with the use of cutting fluids or while dry machining. The present work features a specific study of the application of molybdenum disulphide as solid lubricant for improving tribological properties in turning. An experimental setup has been developed to maintain constant flow rate of solid lubricant powder continuously on to the workpiece and tool interface zone. Further, an analytical expression has also been derived for the estimation of motor speed for achieving desired flow rate. The process performance is judged in terms of cutting force, friction coefficient and the surface finish of workpiece, keeping the cutting conditions constant. The results obtained from the experiment show the effectiveness of the use of the solid lubricant as a viable alternative to dry and wet machining. The unique utility of solid lubricant is highlighted.

Results of long-term, interdisciplinary research of adaptive nano-structured TiAlCrN-based coatings are presented. A number of novel adaptive hard coatings are developed best suited for specific applications associated with ultra-high speed dry machining of hardened tool steels as well as high performance machining of aerospace materials such as nickel-based superalloys and Ti-based alloy.

Adaptation during cutting is a complex process that is related to generation of the surface tribo-films with unique protective/lubricious ability. The tribo-films are acting in synergy with beneficial transformations within the layer of the coatings as well. If all these parameters work together, as a whole, then the coating possesses emergent properties and presents higher ordered surface engineered tribo-system. As such it is capable to sustain strongly varying and intensifying external impacts with unattainable tool life.

Comprehensive investigation of the structural characteristics of the coating is made using XRD, HRTEM, SEM/EDX. Micro-mechanical properties of the coating such as hardness, plasticity index, impact fatigue fracture resistance, low cycling wear resistance are studied at RT and elevated temperatures using Micro Materials Test System. Tribological characteristics of the coatings are investigated vs. temperature in contact with corresponding materials using custom-made high temperature tribometer. Characteristics of the tribo-films are studied in detail using XPS, and EELFAS methods. Tool life is investigated for specific operating conditions. Cutting forces are measured. Wear patterns are indentified. Chips formation is investigated in details as well.

Earlier works proved the excellent adhesion of chemically vapour deposited (CVD) microcrystalline diamond (MCD) to silicon nitride (Si₃N₄) ceramics, due to their good thermal expansion match and chemical compatibility. More recently, nanocrystalline diamond (NCD) coatings were developed aiming at a reduced surface roughness and a lower friction coefficient. Promising results have been obtained with NCD in the machining of hardmetals but at the cost of decreased adhesion levels, mostly due to higher graphitic content and lower deposition temperatures that hinder a strong chemical bonding to the substrate. Thus, the superior adhesion of MCD and the surface properties of NCD were combined in the form of MCD/NCD bilayered coatings for cutting tool performance enhancement in turning operations.

Amorphous carbon (a-C:H) or DLC coatings have been widely utilized in various fields of industries. In particular, DLC-coated tools and dies are utilized in dry stamping with less use of lubricating oils. In this dry tooling, high wear toughness without chipping or cracking is strongly needed for DLC-coated tools in addition to low friction coefficient and specific wear volume. Hence, promising DLC coatings must be selected to satisfy the above requirement for each dry stamping tooling in practice.

In the present paper, nano-laminated coating as well as three different CVD coatings (commercial CVD-DLC, PIG-CVD DLC’s with the interlayer of Ti and CrN), are employed to make feasibility test via dry stamping experiment. To be discussed later, through reduction of clearance between die and punch, this dry stamping experiment l becomes equivalent to a process tribological test in extremely severe conditions.

At first, mechanical and tribological properties of these coatings are investigated. Commercial CVD and PIG-CVD coatings have almost the same hardness (H) and friction coefficient ( m ); e.g. H = 25 – 30 GPa and m = 0.1-0.15. Bimodal thickness ( D L) in nano-lamination is varied by D L = 5 nm and 10 nm. The effect of this D L and layer thickness ration in D L on hardness is experimentally evaluated to demonstrate the controllability of mechanical properties via nano-lamination.

Both WC/Co with the grade of V30 punch and die are prepared for dry stamping test and DLC-coated with different thickness in each above coating. These DLC-coated tools are aligned into a die set with the clearance of 2.5% in the sheet material thickness. Increase of burrs in products in dry stamping test is measured with increasing the number of stamping (N). Using the commercial DLC coating, significant increase of burrs is noticed even after N = 10; no significant increase of burrs is noticed in case of nano-laminated and PIG-CVD coated DLC’s until N = 500 to 700. Engineering durability is evaluated by residual coating area ratio after dry stamping for N = 1000. High durability is attained by nano-lamination with D L = 5 nm. This is because of high hardness by nano-lamination effect and high toughness from chipping and normal crack propagation by lamination.

Crystalline PVD Al₂O₃- coatings offer great potential for their use in cutting operations. They promise high hot hardness and high oxidation resistance at elevated temperatures. Aluminium oxide exists in different crystallographic phases. α-Al₂O₃ appears to be the only thermodynamically stable phase at all common temperatures and pressures. Today there are many efforts to generate α-Al₂O₃ by means of physical vapour deposition. In this regard one problem is the high deposition temperature, which does not allow the deposition on temperature-sensitive materials.

Another promising candidate is γ-Al₂O₃ which is more fine-grained than α-Al₂O₃ and can be deposited at lower temperatures. At high temperatures γ-Al₂O₃ might be transformed into α-Al₂O₃ which could limit the application temperature. But until now it is not clearly proved, up to which temperatures γ-Al₂O₃ thin films are stable and which mechanisms influence the stability. In the present work different (Ti,Al)N/γ-Al₂O₃ are deposited on cemented carbides by means of Magnetron Sputter Ion Plating (MSIP). The (Ti,Al)N bond coat was employed to improve adhesion of γ-Al₂O₃ on the substrate. It could be shown that the γ-phase is stable in vacuum up to 1200°C. In atmosphere the formation of α-Al₂O₃ begins at 900°C and it is influenced by the choice of transition zone between (Ti,Al)N interlayer and γ-Al₂O₃. The results show that the thermal stability of the g-phase and therefore the application temperature of the coating can be enhanced by the choice of interlayer.

The examinations show that γ-Al₂O₃ seems to be suitable for cutting of difficult to machine materials like Inconel or austenitic stainless steel, because high cutting temperatures but also adhesion related sticking of the workpiece material are expected. To prove this several cutting experiments have been done which show (also in comparison to nitride coatings) promising results.

Generation of wear during metal cutting processes affects productivity and manufacturing efficiency, and strongly influences the cutting tool life. Thin hard coatings, synthesized by physical or chemical vapor deposition (PVD and CVD) methods, are then often used in machining processes to enhance wear resistance at the tool-chip interface. The current study investigates the relationship between wear and conditions of the tool-chip contact. In this work, we team numerical simulations to SEM imagery in order to map the tribological parameters such as temperature, contact pressure and friction coefficient within the zone of action of a coated carbide (WC-Co) tool. Several experiments have been conducted on carbide tools with two types of coatings. The first one is made of three layers namely TiN, Al₂O₃ and MT-TiCN, and the second has two layers which are Al₂O₃ and TiCN. For these tests, different cutting speeds and feed rates have been used to investigate the influence of the interface parameters on the chip morphology and wear mechanisms. The cutting speed was taken between 30m/mn and 100m/mn, the speed rate between 0.1mm and 1mm. The analysis shows the effects of tribological parameters (friction, temperature, and contact pressure) on the performance and behavior of coatings under dry contact condition of machining processes.

To minimize tool wear and coating delamination processes, it is necessary to understand the nature of interactions between chip flow, tool and coating materials. In this paper, a hybrid analytical-numerical approach is performed for the orthogonal cutting process. The modelling of the thermomechanical material flow in the primary shear zone, the tool-chip contact length and the sliding-sticking zones is obtained from an analytical approach. In addition, the Finite Element method is used to solve the non linear thermal problem in the chip. At the chip-tool interface, the friction condition can be affected by the important heating induced by the large values of pressure and sliding velocity. In spite of the complexity of phenomena governing the friction law in machining, a reasonable assumption is to consider that at the sliding zone the local friction coefficient is primarily function of the temperature and mechanical parameters (strain, strain rate, stress) at the tool-chip interface.