M. Zlatanović

Deposition of (Ti,Al)N coatings on plasma nitrided steel☆

Abstract In order to meet the requirements of certain special applications, a combined plasma nitriding-coating technology was investigated as a suitable plasma surface treatment. Substrates made of various steel grades were plasma nitrided under carefully controlled process conditions and then coated with a (Ti,Al)N hard coating using a sputter ion plating technique. The structure of the coating depends on the plasma nitriding process parameters used in the surface treatment of the substrate. Since the coating to substrate adhesion is of primary importance for the industrial application of the combined layers, the critical load (as defined by the scratch test method) was measured. The properties of the combined layer were analysed using scanning electron and optical micrographs and by testing the surface and cross-sectional microhardness distribution. The thickness of the hard coating was calculated using calotest ball crater data. The influence of the plasma nitriding and deposition parameters on the combined layer properties is discussed. It is concluded that this combined technology offers new possibilities for the industrial applications of plasma surface treatments.

Effect of plasma nitriding on the properties of (Ti, Al)N coatings deposited onto hot work steel substrates

Abstract The properties of polycrystalline (Ti, Al)N coatings deposited on non-nitrided, classically plasma-nitrided and low pressure plasma-nitrided AISI H11 steel samples were investigated. The plasma and deposition and low pressure plasma nitriding were performed in a Z700-LH magnetron sputter ion plating unit, while a separate unit was used for plasma nitriding of specimens at a pressure of several millibars. The (Ti, Al)N coating was deposited onto all the samples using the same equipment as for the plasma deposition and low pressure plasma nitriding. For the characterization of the composite structures, the following methods were used: scratch test, X-ray diffraction analysis, scanning electron microscopy, scanning tunnelling microscopy and microhardness testing. It was found that plasma nitriding prior to coating deposition strongly affects the growth and properties of hard coatings, such as the microhardness, adhesion, preferred orientation, structure and morphology. Plasma nitriding at low pressures has a greater effect than does conventional plasma nitriding: the adhesion is even more enhanced, the microhardness is increased, the preferred orientation is more pronounced and the structure is still columnar but more compact.

Substrate-induced changes of TiN and (Ti, Al)N coatings due to plasma nitriding*

Abstract Some properties of TiN and (Ti, Al)N coatings deposited on plasma-nitrided samples made of steel grades AISI M2 and AISI H11 were analyzed. In all composite plasma-nitrided/hard-coating layer structures the coating-to-substrate adhesion was enhanced compared with the adhesion of the coatings deposited on non-nitrided specimens. The mechanism of a coating failure during the scratch test is influenced by plasma nitriding of the substrate prior to coating deposition, and the compressive stresses in both TiN and (Ti, Al)N coatings are increased. Substrate-induced preferred orientation growth of both coatings is also influenced by the plasma nitriding, but (111) preferred orientation was found in all cases. All coating structures are found to be columnar and very dense, the coating density being increased due to plasma nitriding of steel substrates. An increase of the coating surface roughness is also due to plasma nitriding of the substrates.

Plasma deposition of (Ti,Al)N coatings at various magnetron discharge power levels

Abstract Properties of (Ti,Al)N coatings, deposited at various magnetron discharge power levels in a double magnetron ion plating system, were investigated. The increase in magnetron discharge power corresponds to an increase in the metal to nitrogen ratio, an increase in the substrate temperature and an increase in the flux of substrate bombarding particles. Microhardness and critical load measurements, scanning electron microscopy (SEM), X-ray and energy dispersive X-ray (EDX) analyses and deposition rate measurements are presented.

Properties of TiN coatings deposited onto hot work steel substrates plasma nitrided at low pressure

Abstract The properties of the surface structure of a plasma nitrided layer beneath a TiN overcoating were investigated. The substrates were AISI H11 grade steel quenched and tempered according to the manufacturers recommendation. Prior to deposition of the coating the substrates were plasma nitrided at a pressure of 5 Pa in a specially designed sputter ion plating unit with an auxiliary anode. The same unit was used for deposition of the TiN coating and plasma nitriding of the substrates. Several substrate induced effects were found to be caused by the previous plasma nitriding of substrates. An excellent coating to substrate adhesion was found by measuring the critical load as defined in the scratch test method. Two modes of coating failure, cohesive and adhesive, were detected and the corresponding critical loads measured. Adhesive failure of the coatings starts at loads in excess of 100 N. The substrate induced preferred orientation of the TiN coating was analyzed by X-ray diffraction of non-nitrided, conventionally plasma nitrided and low pressure plasma nitrided substrates. It is shown that the preferred orientation of coating growth changes from (111) to (220). The conventionally plasma nitrided/TiN coating composite may be considered as intermediate between non-nitrided and low pressure plasma nitrided layers with deposited TiN coating. Compressive stresses were found in both the plasma nitrided layer and the TiN coating.

Pulsed plasma-oxidation of nitrided steel samples

Abstract Plasma nitrided/nitrocarburized samples made of steel grade DIN C15 were post oxidized in a mixtures of H2 and O2 gases using pulse power supply. Plasma nitrocarburizing post-oxidation were performed at 520 °C. The applied process is a combination of plasma nitriding, plasma nitrocarburizing and plasma-oxidation of the compound layer. Pulse plasma was used in all process steps at 5 kHz frequency and pause/pulse ratio 1:19. The XRD examination revealed the existence of the compound zone composed of of the γ′ or γ′+e carbonitride phase with a thin overlayer of magnetite Fe3O4. The architecture of formed surface structure provides the unique mechanical and tribological properties with the diffusion zone responsible for load bearing capacity, wear and fatigue resistance and the double compound layer resistant to wear and corrosion. The results of Vickers microhardness measurements, optical microscopy, XRD and SEM analyses were presented. The obtained surface structures were compared with the samples salt bath nitrided in a conventional industrial unit.

Dynamic Voltage-Current Characteristics of Unipolar Pulse Glow Discharge

Introduction of pulse plasma power supply is one of the major contributions to plasma surface engineering. The voltage and current waveforms of nitrogen and nitrogen/hydrogen glow discharges in the vacuum total pressure range from 0.1 to 15 mbar were recorded and analyzed concerning the shape, typical operating values and the influence of surface treatment process parameters. It was found that the waveform shape of the voltage and current contains information on pulse generator output parameters and vacuum system properties during normal glow, hollow cathode discharge and arcing. We have also performed the scoping of dynamic parameters and static characteristic, in order to obtain information for the generator and chamber modeling, to be used in a future plasma materials treatment processes control. The influence of process parameters such as total pressure, cathode temperature and biasing voltage on current-voltage characteristics was also considered.

Influence of magnetic field configuration on the deposition conditions in an unbalanced magnetron system

Abstract Three configurations of a magnetron sputter ion plating system were considered: the double magnetron open field configuration, single magnetron multipole open field configuration and single magnetron multipole closed field configuration. The (Ti,Al)N coating was deposited in the double magnetron system onto substrates oscillating in and out of high ionization degree plasma obtained by overlapping two magnetron discharges. A strong influence of plasma “density” on the coating growth was found. The experiments were also performed in two single magnetron systems in order to find approximately the region in which a plasma suitable for coating deposition is generated. A rough estimation of high density plasma pattern was given based on voltage-current characteristics of the magnetron discharge. Both single magnetron systems were found suitable for hard coating deposition, but the higher bias current density at the substrate surface had been measured in closed field configuration under similar deposition conditions.

Microstructural modification of TiN deposited by magnetron ion plating : Influence of magnetic field configuration

Abstract Three operating modes of a single magnetron were used during deposition processes: balanced magnetron configuration with no external magnetic field and no bias potential applied (magnetron only), balanced magnetron configuration with permanent magnet cages positioned around the deposition volume to form a closed field configuration (BM mode) and an unbalanced magnetron closed field configuration (UBM mode). The influence of the deposition rate and ion ji to deposited metal atom jm flux ratio ji/jm at the substrate surface on the texture evolution was discussed. The deposition rate distribution over the target to substrate distance was found to depend on magnetic field configuration and also to be different in metallic and reactive mode. In UBM mode it was possible to obtain nearly constant bias current density along the deposition volume at the constant bias potential. The preferred crystalline orientation was changed from (200) to (111) by increasing target to substrate distance in both BM and UBM modes, while the film deposited in magnetron-only configuration preserved (111) texture.

Structural, Mechanical and Optical Properties of TiN and (Ti,AI)N Coatings

The TiN and (Ti,Al)N coatings were deposited onto HSS substrates of different surface roughness in a double magnetron deposition system in order to investigate their optical and some mechanical properties. The spectral reflectivity of the samples was measured in the range from 300-2500 nm, and the coating optical properties were modeled by the screened Drude model and semiclassical model which includes both the intraband and interband electron transitions. It has been demonstrated that the optical reflectivity of the (Ti,Al)N coating may successfully be modeled by the semiclassical relations. Mechanical and structural properties of coatings were investigated by Vickers microhardness measurement, calotest, scratch test, and SEM analysis. One wavelength ellipsometry was applied to measure the optical constants of the TiN coating.