CrAlYCN/CrCN nanoscale multilayer PVD coatings deposited by the combined High Power Impulse Magnetron Sputtering/Unbalanced Magnetron Sputtering (HIPIMS/UBM) technology

CrAlYCN/CrCN coating combining high hardness (H_p = 36 GPa) and low friction coefficient (µ = 0.42 against Al₂O₃) has been developed for machining of Si containing Al-alloys. The coating was deposited by the combined High Power Impulse Magnetron Sputtering/Unbalanced Magnetron sputtering, (HIPIMS/UBM) technology. Macroparticle free Cr⁺ ion flux was generated by HIPIMS discharge to sputter clean the substrates prior to the coating deposition. The use of HIPIMS for surface pre treatment resulted in excellent adhesion, scratch test adhesion critical load value of Lc = 55 N on HSS and Lc = 68 N due to the local epitaxial growth and extremely smooth coating surface, Ra = 0.012 μm due to the elimination of growth defects.The coating crystallised in fcc structure with a preferred {220} orientation. XTEM analysis revealed a nanoscale multilayer structure of the coating with carbon segregated at the column boundaries but also vertically to form a lateral phase at the interfaces between the individual nanolayers.Addition of C to CrAlYN/CrN increased the chemical inertness between cutting tool and workpiece material without deteriorating the oxidation resistance of the coating. Thermo gravimetric analysis showed that the temperature for the onset of rapid oxidation was as high as 940 °C.In dry milling of AlSi9Cu1 alloy, CrAlYCN/CrCN coated 8 mm diameter cemented carbide end mills outperformed non coated end mills by factor of 2.5 with effective hindered built up edge formation mechanism.     

Composition and structure-property relationship of low friction, wear resistant TiN-MoSx composite coating deposited by pulsed closed-field unbalanced magnetron sputtering

Effect of MoS_x content has been studied in TiN-MoS_x composite coating deposited by closed-field unbalanced magnetron sputtering (CFUBMS) using separate MoS₂ and Ti target in N₂ gas environment. Pulsed dc power was applied for both the targets as well as for substrate biasing. Crystallographic orientation and structure of the coating was analysed by grazing incidence X-ray diffraction (GIXRD) technique. The surface morphology and coating fractograph were studied with field emission scanning electron microscopy (FESEM) whereas the composition of the coating was determined by energy dispersive spectroscopy (EDS) by X-ray. Scratch adhesion test, Vickers microhardness test and pin-on-disc test with cemented carbide (WC-6%Co) ball were carried out to investigate mechanical and tribological properties of the coating. Increase in MoS_x content (from 6.22 wt.% to 30.43 wt.%) was found to be associated with decrease in grain size (from 63 nm to 24 nm). Maximum hardness of 32 GPa was obtained for TiN- MoS_x composite coating. Film substrate adhesion was also observed to depend on MoS_x content of the composite coating. Significant improvement in tribological properties was observed. With optimal MoS_x content, it was possible to achieve low friction (µ = 0.02-0.04) and wear resistant (wear coefficient = 5.5 × 10^− 16 m³/Nm) composite solid lubricant coating.     

Adaption of graded Cr/CrN-interlayer thickness to cemented carbide substrates' roughness for improving the adhesion of HPPMS PVD films and the cutting performance

High Power Pulsed Magnetron Sputtering (HPPMS) techniques jointly with the deposition of a graded Cr/CrN-nanointerlayer on cutting inserts can increase the film adhesion and consequently the tool life. These improvements depend on the roughness of the employed cemented carbide substrates. The investigations described in the present paper intend to explain the effect of Cr/CrN-interlayer thickness and substrate roughness on the coating adhesion and cutting performance. To attain various roughnesses, the applied cemented carbide inserts were superficially treated. These treatments were grinding at a medium roughness level, or grinding with subsequent polishing for enhancing the surface integrity and finally, in all cases, micro-blasting by fine Al₂O₃ grains. After Ar-ion etching, graded Cr/CrN adhesive layers with different thicknesses were deposited by HPPMS technology on the variously pretreated substrates. Subsequently, an approximately 3 μm thick (Ti,Al)N film was deposited by HPPMS PVD on all used inserts. Rockwell C indentations and inclined impact tests were performed to assess qualitatively and quantitatively the films' adhesion. The cutting performance of the coated tools was investigated in milling of 42CrMo4 QT. FEM supported calculations of the developed stresses during the material removal process contributed in explaining the obtained tool wear results. In these calculations, the adhesion, dependent on the substrate roughness characteristics and on the adhesive interlayer thickness, was taken into account. The results revealed that the effectiveness of HPPMS adhesive graded Cr/CrN-nanointerlayer strongly depends on the substrate surface integrity and on the interlayer thickness. Thus, the film adhesion and consequently the cutting performance can be significantly improved if the interlayer thickness is adapted to the substrate roughness.     

Evaluation of vanadium carbide coatings on AISI H13 obtained by thermo-reactive deposition/diffusion technique

Vanadium carbide coatings on AISI H13 steel were prepared by thermo-reactive deposition/diffusion process (TRD) in molten salt bath for 1 to 6 h at 920 °C and 1000 °C, respectively. The obtained coatings were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and X-ray diffraction analysis (XRD). Equiaxed grains were observed throughout the coatings. The grain size gradually increased from the coating/substrate interface to the top surface. The coatings were composed of ordered state V₆C₅ phase and disordered state VC_x (x = 0.83-0.88) phases and had a preferential orientation of (111) and (200) planes. The values of nano-indentation hardness and elastic modulus of the coating are 28.1 ± 0.7 GPa and 421 ± 14 GPa, respectively. The growth of the vanadium carbide coating by the TRD process followed a parabolic kinetics with an activation energy of 199.3 kJ/mol. The variation of the coating thickness on the AISI H13 steel with treating time and temperature can be determined.     

Characterization and blood compatibility of TiCxN1 − x hard coating prepared by plasma electrolytic carbonitriding

A novel technique to form Ti(C, N) on titanium, named as plasma electrolytic carbonitriding (PEC/N) on cathode was successful used to prepare TiC_xN_1 − x coating. The structure, composition and morphology of the coating were characterized by XRD, XPS and SEM, respectively. The results indicated that TiC_0.3N_0.7 as a new species appears on the surface of the titanium plate, and the thickness of the coating with porous surface morphology increases with the treated time. The blood compatibility of the TiC_0.3N_0.7 coating was evaluated by haemolysis ratios, dynamic blood clotting test, plasma recalcification time and platelet adhesion. The results indicated that the blood compatibility of the plasma-treated titanium with TiC_xN_1 − x coating is significantly improved as compared to the original titanium. Additionally, the results derived from measurements of hardness and corrosion indicated that the coating has excellent mechanical and corrosion-resistant properties.     

Rapidly solidified non-equilibrium microstructure and phase transformation of plasma cladding Fe-based alloy coating

The rapidly solidified microstructural and compositional features, the precipitation and transformation of carbides during aging of Fe-based alloy coating prepared by plasma cladding have been investigated. The clad coating materials, whose powder mixture of Fe, Cr, Ni, B, Si and C with a weight ratio of 54.5:35:5:1:2:2.5, is processed using a non-transferred plasma arc. The clad coating adheres with low carbon steel in a good metallurgical bonding and the rate of dilution is 15-20%. Microstructural studies demonstrate that the coating possesses the metastable microstructure comprising the primary dendritic γ-austenite which is a non-equilibrium phase with an extended solid solution of alloying elements and interdendritic eutectic consisting of γ-austenite and (Cr,Fe)₇C₃ carbides. During the high temperature aging at 1253 K for 2 h, the fine spherical (Cr,Fe)₂₃C₆ carbide nucleates within (Cr,Fe)₇C₃ carbide and austenite matrix, and some martensite (α) also forms during cooling. The solidification and evolution sequence of the phase can be represented as follows: L → γ + L → γ + (γ + (Cr,Fe)₇C₃) → (γ + (Cr,Fe)₂₃C₆ + α). Due to the precipitation of (Cr,Fe)₂₃C₆ carbide and uniform distribution of carbide in the as-aged coating, the average hardness becomes higher than that of the as-clad coating.     

An investigation on the microstructure and oxidation behavior of laser remelted air plasma sprayed thermal barrier coatings

NiCrAlY bond-coat was coated on Inconel 718 substrate by air plasma spraying (APS) followed by APS ZrO₂-8 wt.%Y₂O₃ as top-coat. Using CO₂ laser of different energy densities, ceramic top-coat surface was remelted. Laser remelting with high energy density (4 J/mm²) produced a dense microstructure over the whole thickness of top-coat, while low energy density (0.67 J/mm²) laser remelting produced a ~ 50 μm thick dense layer on the top-coat surface. It was found that the volume fraction of monoclinic phase decreased from 9% in as-sprayed coating to 4% and 3% after laser remelting with high and low energy density respectively. After isothermal oxidation at 1200 °C for 200 h, the thickness of oxide layer (TGO) in the sample produced by low energy density laser remelting was ~ 5.6 μm, which was thinner than that of oxide layer in as-sprayed (~ 7.6 μm) and high energy density laser remelted (~ 7.5 μm) samples. A uniform and continuous oxide layer was found to develop on the bond-coat surface after low energy density laser remelting. Thicker oxide layer containing Cr₂O₃, NiO and spinel oxides was observed in both as-sprayed and high energy density laser remelted coatings. After cyclic oxidation at 1200 °C for 240 h, the weight gain per unit area of as-sprayed coating was similar to that of high energy density laser remelted coating while a significantly smaller weight gain was found in low energy density laser remelted coating.     

The influence of age-hardening on turning and milling performance of Ti-Al-N coated inserts

Ti-Al-N coatings are well known for their excellent properties and age-hardening abilities. Here we show that the life-time of coated inserts during turning of stainless steel can be increased to 200% by post-deposition vacuum annealing at 900 °C combined with a ~ 1 K/min vacuum furnace cooling. During milling of 42CrMo steel an increase in tool life-time to 140% is only obtained if the cooling condition after annealing at 900 °C contains a fast segment with 50 K/min from 800 to 700 °C. Thereby, the Co-binder in cemented carbide exhibits a retarded phase transformation from cubic to hexagonal. Consequently, the fracture toughness of the cemented carbide is reduced only from ~ 10.8 to 10.4 MPa√m while the coating still has an adhesive strength of ~ 65 N.Our results indicate that best machining performances of coated inserts are obtained after annealing at 900 °C where the supersaturated Ti_0.34Al_0.66N coating undergoes spinodal decomposition to form nm-sized cubic TiN and AlN domains resulting in a hardness increase from 34.5 to 38.7 GPa. Additionally, we demonstrate that careful attention needs to be paid on the influence of annealing conditions on adhesive strength and fracture toughness of coated inserts.     

CrAlYN/CrN superlattice coatings deposited by the combined high power impulse magnetron sputtering/unbalanced magnetron sputtering technique

CrAlYN/CrN coatings represent a new generation Ti-free PVD coatings tailored to serve high temperature applications such as dry high speed machining and protection of special grades aerospace and automotive alloys against environmental attack. The novel High Power Impulse Magnetron Sputtering (HIPIMS) technique was used for substrate pre-treatment (etching) followed by coating deposition utilising Unbalanced Magnetron Sputtering (UBM). The employment of HIPIMS resulted in smooth (R_a = 0.036 μm) and well adherent films with typical scratch adhesion critical load values on M2 high speed steel of L_C = 65 N. Low-angle XRD analysis showed that the coating has a nanoscale multilayer (superlattice) structure with a typical bi-layer thickness of 4 nm. XTEM observations confirmed this result and further revealed the dense, growth defect free structure of the coating due to the HIPIMS etching. CrAlYN/CrN combines high hardness of HK_0.025 = 3320 with a low coefficient of friction of 0.5 and an exceptionally low sliding wear coefficient of 3.7 × 10^− 17 m³ N^− 1 m^− 2, which is comparable to that of TiAlN/VN and Me-Carbon films. In dry high speed milling (V_cutting = 385 m min^− 1) of hardened A2 tool steel (HRC = 58), 8 mm cemented carbide ball nosed end mills coated with CrAlYN/CrN outperformed TiAlCrYN, which is one of the market leading coatings dedicated to this application. When the test is carried out at the higher end of the cutting speed range of 500 m min^− 1 this difference in performance becomes even more pronounced (factor of 8 longer life time), which demonstrates the excellent quality of CrAlYN/CrN.     

Effect of laser irradiation on failure mechanism of TiCp reinforced titanium composite coating produced by laser cladding

Laser cladding is an effective technique to coat a metallic substrate with a layer of a different nature. It has been widely reported that the most important combined parameters controlling the quality of the coating are the specific energy (E) and the powder density (Ψ). In the present work, clad deposits of Ti6Al4V + 60 wt.% TiC were prepared on a Ti6Al4V substrate using an optimum combination of E_c = 24 J/mm² and ψ_c = 3 mg/mm². These experiments were performed using a laser power of 400 and 600 W, in order to study the effect of laser power on the properties of the clad. The microstructure, phase composition and nanohardness of the coatings were investigated by optical microscopy, scanning electron microscopy and X-ray diffraction. During laser processing, TiC can be partially converted to TiC_X (X = 0.5) due mainly to the TiC dissolution into the laser-generated melting pool and subsequent precipitation during cooling. It was observed that the lower laser power limit reduces primary TiC dissolution but it also promotes secondary carbide alignment at the interface. On the other hand, the damage mechanism induced by high laser power is dominated by primary TiC particle cracking by the high stress concentration at the particle–matrix interface followed by ductile failure of the matrix. It is also remarkable that irradiance affects the TiC/TiC_x ratio despite E_c and ψ_c are fixed and it determines hardness distribution inside the coating.     

Preparation and oxidation protection of CVD SiC/a-BC/SiC coatings for 3D C/SiC composites

An amorphous boron carbide (a-BC) coating was prepared by LPCVD process from BCl₃-CH₄-H₂-Ar system. XPS result showed that the boron concentration was 15.0 at.%, and carbon was 82.0 at.%. One third of boron was distributed to a bonding with carbon and 37.0 at.% was dissolved in graphite lattice. A multiple-layered structure of CVD SiC/a-BC/SiC was coated on 3D C/SiC composites. Oxidation tests were conducted at 700, 1000, and 1200 °C in 14 vol.% H₂O/8 vol.% O₂/78 vol.% Ar atmosphere up to 100 h. The 3D C/SiC composites with the modified coating system had a good oxidation resistance. This resulted in the high strength retained ratio of the composites even after the oxidation.     

Microwave atmospheric pressure plasma for surface treatment and reactive coating on steel surfaces

The microwave atmospheric plasma technology developed at DANA corporation has previously been used for various heat treatment applications. In this paper, we demonstrate the use of atmospheric plasma assisted chemical vapor deposition (APACVD) process to coat steel surfaces. The method described in this paper involves two steps. The first step is to coat a Si based interlayer and then to coat C either by CH₄ or C₂H₂ containing plasma. At the end of the coating process, a thick, colorful, reactive coating is formed on the steel surface. The XRD and compositional analyses have established the presence of metal carbide phases such as Cr₇C₃, Fe_15.1C in these coatings. These phases are well bonded to the steel surface with no separate interlayer formation. Hardness tests show that carbide phases formed in a nitrogen containing plasma are 2.5 times harder compared to the bulk. The faster deposition rates at atmospheric pressure clearly indicate that this process is not only rapid (∼ 0.2 μm/min) but also cost-effective.     

The corrosion behavior of plasma sprayed Fe2Al5 coating in molten Zn

Aluminum coating was plasma sprayed on Fe-0.14-0.22 wt.% C steel substrate, and heat diffusion treatment at 923 °C for 4 h was preformed to the aluminum coating to form Fe₂Al₅ inter-metallic compound coating. The corrosion mechanism of the Fe₂Al₅ coating in molten zinc was investigated. SEM and EDS analysis results show that the corrosion process of the Fe₂Al₅ layer in molten zinc is as follows: Fe₂Al₅ → Fe₂Al₅Zn_x (η) → η + L(liquid phase) → L + η + δ(FeZn₇) → L + δ → L. The η phase and the eutectic structure (η + δ) prevent the diffusion of zinc atoms efficiently. Therefore the Fe₂Al₅ coating delays the reaction between the substrate and molten zinc, promoting the corrosion resistance of the substrate.     

Wettability and machinability study of pure aluminium towards uncoated and coated carbide cutting tool inserts

An investigation has been conducted to study wetting characteristics of aluminium towards different cutting tool materials for assessing the compatibility for dry machining of aluminum. For this purpose uncoated carbide (94%WC + 6%Co) and mono or multi-layer coated carbide tools with top coating of TiC, TiN, Al₂O₃ and diamond have been used. It was observed that aluminium had a tendency to wet uncoated carbide (94%WC + 6%Co) inserts. However, wetting was more pronounced when surface was enriched with cobalt. In contrast, wetting of aluminium was less when the WC content of the carbide tool surface increased. Coatings like TiC, TiN or Al₂O₃ could not show pure non-wetting characteristics for aluminium. The aluminium appeared to dissolve the coatings in different degrees. On the other hand, coating of diamond exhibited inertness towards aluminum leading to non-wetting behaviour. Turning test with aluminium indicated heavy material built up on uncoated (94%WC + 6%Co) tool. Built up edge formation could not be avoided when carbide inserts with a top coating of TiC, TiN, Al₂O₃ were engaged in machining of aluminium. However, the non-wetting characteristic of diamond coating was reflected during machining of aluminium. The chips slided smoothly over the rake face leaving no trace of edge built up.     

Effects of low temperature thermal treatment on zinc and/or tin plated coatings of AZ91D magnesium alloy

A new method was investigated to obtain composite coatings on the AZ91D magnesium alloy by electrodeposition and low temperature thermal treatment. Zinc and tin were introduced to AZ91D Mg alloy surface by electroplating firstly. And a succedent thermal treatment was carried out at 190 ± 10 °C for 12 h. The surface and cross-section morphologies of the plated coatings with and without thermal treatment were studied by scanning electron microscopy (SEM). And the microstructure was determined by X-ray diffraction (XRD). The results reveal that it was difficult to obtain good adhesion plated Sn coating but easy to get well-adherent plated Zn coating. And the thermal treatment promoted the formation of Mg₂Sn in the plated Sn coating and the recrystallization in the plated Zn coating. The plated double Zn-Sn coating owned good adhesion and uniform surface. Furthermore, when the plated double Zn-Sn coating was treated at 190 ± 10 °C for 12 h, a three-layer structure coating was formed due to the diffusion of tin. The results of the anodic polarization behaviors in 5 wt.% NaCl solution show that the three-layer structure coating could provide better protection for AZ91D substrate than the plated Zn-Sn coating.     

Enhanced corrosion resistance of WC-Co with an ion beam mixed silicon carbide coating

Strong adhesion of a silicon carbide (SiC) coating to a WC-Co substrate was achieved through an ion beam mixing technique and the corrosion resistance of the SiC coated WC-Co was investigated by means of a potentiodynamic electrochemical test. In a 1 M NaOH solution, the corrosion current density of SiC-coated WC-Co after heat treatment at 500 °C was about 50 times lower than that for the as-received WC-Co. In addition, the corrosion resistance systematically increases with increasing the SiC coating thickness. On the other hand, for a 0.5 M H₂SO₄ solution, the corrosion current density for SiC-coated WC-Co was about 3 times lower than that for the as-received WC-Co. We discuss the physical reasons for the changes in the corrosion current density with the different electrolytes.     

Substrate temperature influence on boron carbide coatings grown by the PLD technique

Boron carbide films were synthesized by laser ablation technique, using a target of B₄C with 99.9% of purity, varying the substrate temperature between room temperature and 650 °C, in order to produce the hexagonal phase (h-BC). Films were grown on (111)-silicon wafers in an ultra high vacuum system with a base pressure in the order of 10^− 7 Pa. For the films' growth, an atmosphere of (CH₄) at a pressure of 2.5 Pa was used. During the process, the substrate temperature was varied in order to identify the influence of this parameter on the coatings' structure, composition and morphology. XRD analysis did not present peaks of BC, possibly because of the amorphous character of the film that has different phases. Films were characterized by several techniques as in situ Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and ex situ electron diffraction. Results present a concentration of 50 at.% for the sample grown to 650 °C. Electron diffraction showed an interplanar spacing (d₍₀₀₂₎ = 0.334 nm) and also other hkl reflections have been identified. Lattice parameters calculated from the interplanar spacing a = 0.585 nm and c = 1.2 nm obtained for the sample grown at 650 °C are similar to those reports for hexagonal boron carbide.     

Improvement of the oxidation and wear resistance of pure Ti by laser cladding at elevated temperature

NiCrBSi and NiCrBSi/WC-Ni composite coatings were produced on pure Ti substrates by the laser cladding technology. Thermal gravimetric (TG) analysis was used to evaluate the high temperature oxidation resistance of the laser cladding coatings. The friction and wear behavior was tested through sliding against the Si₃N₄ ball at elevated temperatures of 300 °C and 500 °C. Besides, the morphologies of the worn surfaces and wear debris were analyzed by scanning electron microscopy (SEM) and three dimensional non-contact surface mapping. The results show that the microhardness, high temperature oxidation resistance and high temperature wear resistance of the pure Ti substrates are greatly increased. For the pure Ti substrate, the wear behavior is dominated by adhesive wear, abrasive wear and severe plastic deformation, while both laser cladding coatings, involving only mild abrasive and fatigue wear, are able to prevent the substrates from severe adhesion and abrasive wear. In particular, the laser cladding NiCrBSi/WC-Ni composite coating shows better high temperature wear resistance than the NiCrBSi coating, which is due to the formation of a hard WC phase in the composite coating.     

Development of micro milling tool made of single crystalline diamond for ceramic cutting

In order to machine micro aspheric ceramic molds precisely and efficiently, micro milling tools made of single crystalline diamond (SCD) are developed. Many cutting edges are fabricated 3-dimensionally on the edge of a cylindrical SCD by a laser beam. Flat binderless tungsten carbide mold was cut with the developed tool to evaluate the tool wear rate and its life. Some micro aspheric molds of tungsten carbide were cut with the tool at a rotational speed of 50,000 min⁻¹. The molds were cut in the ductile mode. The form accuracy obtained was about 100 nm P–V and the surface roughness 12 nm R_z.     

Microstructure and tribological behaviour of nano-structured metal matrix composite boride coatings synthesized by combined laser and sol-gel technology

Hard nano-structured metal matrix composite (MMC) boride coatings have been synthesized by laser melting of pre-placed powder mixture paste of B₄C + sol-gel derived nano-particulate TiO₂ on AISI 1050 (EN43) medium carbon steel and AISI 316L stainless steel substrates. Different coating/processing gas conditions were employed to understand the influence of graphite and nitrogen gas interactions with the coating material at high temperatures. Laser synthesized coatings were characterized by SEM, EDX, FEGSEM, XRD and HRTEM. Results show that it is possible to synthesize nano-structured MMC coatings (with TiB₂ and TiB particulates in the ranges of 5-10 nm, 20 nm and 200-500 nm) by employing the combined laser and sol-gel route. Nano-particulate and sub-micron level TiB and TiB₂ are found dispersed throughout the metal matrix. Other borides and carbides are present in micro-level patches dispersed in a eutectic matrix. Hardness of the composite coatings is in the range 800-2000 HV_0.1. The minimum coefficient of sliding friction obtained in a pin-on-disc set-up was 0.35 (against cemented tungsten carbide) while wear rates (against diamond) were substantially improved (up to 5 fold reduction) over that of the substrates.