ICMCTF2007 Session B5-2: Properties and Characterization of Hard Coatings and Surfaces

PVD process parameters such as deposition duration, bias voltage etc., affect significantly film strength properties and through them the coated tool cutting performance. In the described investigations, cemented carbide inserts were coated with nanocomposite (Ti₄₅Al₅₅)N films through high ionization sputtering (HIS) PVD processes by plasma booster technology, keeping all process parameters constant in the vacuum chamber except the deposition time.

To compare the cutting performance of inserts of the same TiAlN PVD films and coating thickness, the film deposition was performed as follows. Cemented carbide specimens were placed in various locations in the vacuum chamber simultaneously with further items to be coated, so that due to shadowing effects the plasma flux was variable. As a result in PVD processes of an overall duration of 14400 sec and of 5400 sec respectively, inserts with various thicknesses were produced according to their location in the vacuum chamber. Coated cutting inserts with a film thickness of approximately 2.7 µm, originated from both the aforementioned PVD processes, with various overall deposition times were employed.

The characterization of the films' strength properties was performed through nanoindentation and a finite element methods (FEM) supported evaluation of the corresponding measurement results. Impact tests were carried out to determine film fatigue properties. The cutting performance of the coated inserts was investigated systematically in milling. The obtained results were explained by FEM simulation of the cutting process, considering the mechanical and fatigue properties, established by nanoindentations and impact tests as described before. The results revealed that longer deposition times improve the yield, rapture and critical fatigue stress, resulting to a significant tool life increase.

TiN/Si₃N₄, TiFeN and TiFeN/Mo films were produced on silicon substrates using a dual ion beam system. Mechanical properties have been determined by nano-indentation and tribological properties have been measured by nano-scratch testing. The nano-indentation showed that harder nitrides exhibited higher ratios of hardness to modulus (H/E). The dependence of the resistance to plastic deformation (H³/E²) on hardness was approximately linear.

When applying a load, which increases linearly with distance, the distance to the start of cracks (first cohesive failure) l_c1 and the distance to total delamination lc₂ can be expressed as a ratio T = (lc₂ - lc₁)/lc₁. The ratio can be treated as a measure of the specific coating/substrate toughness (SCT). This SCT is seen to correlate well with the ratio H/E. The SCT appears to give a better correlation with H/E than the crack propagation resistance (lc₂ - lc₁) x lc₁ defined earlier by Zhang at al.¹

It is shown by our data that use of the SCT enables comparison of data on films of different thickness and/or using probe of differing radius. Data from a wide range of different film-substrate systems have been evaluated and correlations with H/E investigated and optimum H/E values for enhanced durability determined.

¹ Zhang S, Sun D, Fu Y and Du H, Thin Solid Films 447-8, 462 (2004)}

Today cemented carbides are the dominant tool material for metal cutting using indexable inserts. For these, coatings of hard refractory materials such as TiC, Ti(C,N), TiN and Al₂O₃, are frequently used in order to further enhance the performance of the cutting tool insert.

In the machining of steel (iron based metals) Al₂O₃ showing a high chemical and thermal stability, plays an important role acting as a diffusion barrier protecting the underlying cemented carbide from dissolution thus reducing the crater wear on the tool rake face. However, due to the sliding of work material against the rake face the relatively soft Al₂O₃ is exposed to both adhesive and abrasive wear which limits its lifetime in many cutting applications and consequently attempts have been made to improve the properties of CVD Al₂O@₃coatings, e.g. through the deposition of texture-controlled CVD Al₂O₃coatings.

The aim of the present study is to investigate the abrasive wear resistance of CVD Al₂O₃ coatings with different textures, i.e. <0112>, <1014> and <0001> and growth textures, using a micro-abrasion test with diamond particles as the abrasive medium. All CVD Al₂O₃coatings where deposited on cemented carbide substrates with an intermediate Ti(C,N) layer and the as-deposited coatings were characterised using X-ray diffraction and scanning electron microscopy. The results are discussed in relation to the results obtained from continuous turning tests using a quenched and tempered low alloyed steel, AISI 1045, and cast iron, AISI A48-45B, as work materials. The identified wear mechanisms of the coatings have been identified using scanning electron microscopy and energy dispersive X-ray spectroscopy. The potential of the micro-abrasion test in the tribological characterisation of thin CVD coatings for cutting tool applications is discussed.