Effect of Bias Voltage on Microstructure and Mechanical Properties of Nanocomposite AlCrN / TiSiN Coatings

Liang Xiaobiao, Chen Yafen, Zhang Jiaquan

Guangdong Huasheng Nanotechnology Co.,Ltd.

Abstract: Multi-layer AlCrN / TiSiN composite coating film with functional layer biases of 40V,80V,120V and 150V are prepared by AIP+HiPiMS technology.Surface droplets are observed by SEM,coating hardness,coating adhesion and stress are measured,and cutting tests are conducted to explore the impact of functional layer bias changes on coating performance.It is found that as the bias voltage of the functional layer increases,the number of droplets decrease first and then increase,with the hardness and adhesion increase and then decrease.The hardness and adhesion reach their maximum at 120V,reaching 34.6GPa and 117.5N respectively.The internal stress increases with the increase of substrate bias of the functional layer.The cutting test results show that the longest service life is 8 hours with the 120V functional layer bias．

Keywords: functional layer bias; surface droplets; nano hardness; adhesion;cutting performance

1 Introduction

Hard coating can effectively hinder the diffusion of elements between tools and cutting materials and improve the service life of machining tools [1,2].CrN coating has excellent corrosion resistance and wear resistance,and its internal stress is very low,but its thermal stability is poor when it exceeds 650℃ [3].After introducing Al atoms into CrN and replacing part of Cr elements,Al elements and Cr elements form dense Al2O3 and Cr2O3 films at high temperature,which effectively prevents O elements from diffusing into the coating,and the oxidation resistance of the coating will increase with the increase of Al element content [4,5].However,when the temperature is higher than 900℃,brittle w-AlN [6] will precipitate in AlCrN coating,which will deteriorate the coating performance,especially during high-speed dry cutting.The temperature in the contact area between the tool and the chip can reach up to 1000℃,which deteriorates the cutting performance of the tool and makes it difficult to meet the needs of modern cutting.

Some studies show that [7,8],the introduction of Si into AlCrN coating can greatly improve the oxidation resistance of the coating,and at the same time,amorphous SiNx-coated nanocrystalline structure is formed in the coating,which reduces the grain size and improves the strength and toughness of the coating.Parlinska-Wojtan M.et al.[9] added Si element to TiAlN to prepare TiAlSiN coating.With the addition of Al+Si,the hardness of the coating is up to 41GPa,which is higher than that of TiAlN coating.Hsien-Wei Chen et al.[10] found that the coating grains were refined and the oxidation resistance was improved by adding Si to AlCrN coating.The design of multi-layer structure has been proved to be an effective method to improve the coating performance [10,11].The strong interface between layers can hinder the dislocation movement in the coating,inhibit the growth of columnar crystals and hinder the crack propagation,thus improving the thermal stability,hardness after high-temperature treatment and high-temperature oxidation resistance of the coating.The AlCrN/TiSiN multi-layer coating combines the excellent oxidation resistance of AlCrN coating and the superhard characteristics of TiSiN coating,and has broad development prospects [12,11]

In the preparation process,high-density ions and neutral particles are deposited on the surface of the substrate under the bias of the substrate,but the phenomenon of large particle droplets will inevitably exist in the deposition process of AIP.However,magnetron sputtering can obtain good surface quality in the deposition process,but the ionization rate and deposition rate are low,so relatively excellent coatings can be prepared by combining AIP and magnetron sputtering.However,different bias voltage will cause the plasma energy to change in a large range,which will affect the surface morphology,microstructure and mechanical properties of the coating.Therefore, based on the preparation of AlCrN/TiSiN multilayer structure coatings using arc ion plating(AIP) and high-power pulsed magnetron sputtering (HiPiMS) composite technology, this article adjusts the negative bias of the functional layer to explore the effects of changes in the negative bias of the functional layer on the microstructure, coating composition, and mechanical properties of the multilayer AlCrN/TiSiN..

2 Experimental equipment and scheme

The vacuum coating equipment of HA800 magnetron sputtering and AIP by Guangdong Huasheng Nanotechnology Co.,Ltd.was used,and PVD coating was deposited by using six circular cathode arc targets and two square magnetron targets.The diameter of the circular target was 160mm,the atomic ratio of Al/Cr was 70 /30,the size of the square target was 500mm ×100mm×10mm,and the atomic ratio of Ti/Si was 80 /20.Polished cemented carbide square specimen,stainless steel specimen and APMT1135PDER cemented carbide blade were used for analysis and testing,and four different negative bias voltages (40V,80V,120V,150V) were applied to the substrate.

The coating preparation process includes thermal radiation heating,plasma etching,coating and cooling.Plasma etching uses Ar+ ionized by argon to etch by applying a voltage of -200V on the substrate,and plasma etching cleans for about 45min minutes.Coating process: the bottom layer is about 0.4μm AlCrN coating prepared by AIP,and the bias voltage is fixed;The functional layer is about 0.14μm AlCrN and 0.13μm TiSiN alternately deposited for 6 times,and the surface layer is TiSiN coating.AlCrN was coated in a mixed atmosphere of nitrogen and argon,and the chamber pressure was kept at 2.8 Pa,and the target current was about 190A;TiSiN was prepared by HiPiMS technology,with a fixed bias,power of about 8kW,duty cycle of about 28%,cavity pressure of about 0.6 Pa,and Ar:N2 ratio of about 3:1,in which the bias of AlCrN coating in the functional layer was 40V,80V,120V and 150V respectively.The coating temperature is kept at 500℃,and the overall thickness of the coating is about 2.2μm..

Field emission Zeiss FESEM Sigma300 scanning electron microscope was used to analyze the microstructure of the coated sample,and the structure,surface morphology and coating thickness of the coating were characterized.Anton Paar NHT3 nano-indentation tester was used to measure the hardness of the coating.20mN load was applied,the loading rate was 40mN/min,and the load was kept for 10s.The indentation depth was less than 1/10th of the thickness of the coating,and sufficient data was collected through matrix selection.Anton Paar NST3 nano-scratch tester was used to measure the adhesion between coating and cemented carbide substrate,and 120N termination load was set,the scratch length was 3mm,the scratch time was 30s,and each coating was repeated three times.Using SuPro FST1000 film stress meter,based on the principle of substrate bending method and Stoney equation,the film residual stress of stainless steel substrate was tested,and the variation trend of residual stress in different schemes was compared by taking relative values.

Using APMT1135PDER milling inserts for cutting tests, 45 steel was cut on the K-V855GCNC CNC milling machine of Nuojin Machinery, using commonly used processing parameters in the industry.Using the Keyence VHX-7000 digital microscope system to capture the wear status during the cutting process, the coating performance is evaluated by comparing the wear amount on the back face of the milling cutter. The standard for judging the service life of the milling insert is that the flank wear reaches 300μm.

3 Experimental results and discussion

3.1 Effect of negative bias of functional layer on surface morphology and cross-sectional size of coating

As shown in Figure 1,the change of the negative bias of the functional layer will lead to the change of the droplets of the functional layer,and the subsequent surface deposition will show the same droplet results of the functional layer.It can be seen that with the increase of the bias,the number of droplets on the coating surface will first decrease and then increase.When the negative bias voltage of the functional layer is 40V,the number and size of droplets are large;When the negative bias of the functional layer increases to 80V and 120V,the droplets on the surface of the coating decrease,which is because the negative bias of the substrate accelerates the positive ions in the plasma,and the energy obtained by the acceleration increases with the increase of the negative bias[14].The energetic particles bombard the coating more strongly,and the droplets on the surface of the functional layer decrease,which is manifested as the decrease of coating droplets in the subsequent surface coating deposition process.When the negative bias voltage of the substrate continues to increase to 150V,the ion bombardment energy is strong,which leads to the reverse sputtering of the coating,and even the formation of holes on the surface of the functional layer coating.When the subsequent coating is deposited,more droplets will remain at the holes.

Fig.1 droplet morphology on coating surface under negative bias of different functional layers

Figure 2 shows the cross-sectional morphology and coating thickness statistics of different functional layers under negative bias.It can be seen that with the increase of the negative bias voltage of the functional layer,the cross-sectional thickness decreases gradually,from 2.303μm at 40V to 2.021μm at 150V.The main reason is that with the increase of the negative bias voltage of the functional layer,the structure of the coating functional layer becomes dense,which leads to the decrease of the coating thickness,especially at 150V,the anti-sputtering ability is enhanced, which further reduces the cross-sectional coating thickness.

Fig.2 Influence of negative bias voltage of different functional layers on cross-section thickness of coating.

3.2 Effect of negative bias of functional layer on internal stress and hardness of coating

Fig.3a shows the change of internal stress curve of coating under negative bias of different functional layers.It can be seen that the coating stress is compressive stress,and the internal stress of the coating gradually increases with the increase of bias,from 40V of 2.23GPa increased to 4.35 GPa of 150V .

(a) Effect of negative bias on coating stress

(b) Effect of negative bias on coating hardness

Fig.3 Effect of negative bias of different functional layers on residual stress and hardness of coating.

Fig.3b shows the change of coating hardness under negative bias of different functional layers.When the negative bias voltage of the functional layer is 40V,the nanoindentation hardness is 33.2GPa,and when the negative bias voltage of the functional layer is increased to 80V,the nanoindentation hardness increases to 34.2 GPa.When the negative bias voltage of the functional layer continues to increase to 120V and 150V,the nanoindentation hardness is 34.6 GPa and 34GPa,respectively,and the hardness first increases and then decreases.The change of coating hardness is related to the negative bias of functional layer.With the increase of negative bias of functional layer,the bombardment energy increases during preparation,which makes the coating change from columnar crystal to fine-grained structure.At the same time,the residual compressive stress hinders the grain boundary slip of columnar crystal and improves the deformation resistance of the coating.Therefore,when the negative bias of functional layer increases from 40V to 120V,the nano-indentation hardness increases,but when the negative bias of functional layer continues to increase to 150V,the coating crystal structure is destroyed,and the reverse sputtering is enhanced.

3.3 Influence of negative bias of different functional layers on coating adhesion

Fig.4 shows the coating adhesion under negative bias of different functional layers,and the Lc1 coating adhesion is evaluated based on the matrix where the coating begins to appear.The Lc1 of 40V,80V,120V and 150V are 98.95N,109.8N,117.5n and 93N.It can be seen that with the increase of negative bias voltage of functional layer from 40V to 120V,the stress and hardness of the coating increase,and the bonding force of the coating also increases.When the bias voltage continues to increase to 150V,the internal stress is higher,resulting in greater brittleness of the coating and lower bonding force.

Fig.4 Influence of negative bias of different functional layers on coating adhesion.

3.4    Influence of negative bias of different functional layers on cutting performance

The cutting test conditions are linear velocity Vc = 117.5m/min,feed fz = 0.57mm/r,cutting depth ap = 0.3mm,and cooling condition is air cooling.Figure 5 shows the relationship between cutting service life and flank wear.

Fig.5 Wear comparison of different functional layers under negative bias under different cutting service life conditions.

When the negative bias of 40V functional layer was processed for 5 hours,the flank wear reached 0.41 mm and failed,while when the negative bias of 40 V functional layer reached 0.36 mm and failed at 6 hours,when the negative bias of 120V functional layer was processed for 8 hours,the wear on the rear cutting surface is 0 33mm.When the bias voltage is 150V and the cutting tool is 0.33mm,the cutting tool fails after 6 hours,and the wear of the flank is 0.36mm.The trend of cutting tool failure is consistent with the change trend of coating hardness,and the negative bias voltage of the functional layer is 120V,which shows the best wear resistance.

Fig.6 shows the comparison of tool wear patterns at the processing time of 5h.It can be seen that when the functional layer is negatively biased at 40V,the coating has been peeled off at the cutting edge because of its low hardness and stress,and the cutting edge of the tool has collapsed because the matrix participates in cutting after the coating peeling.Increasing the negative bias voltage of the functional layer to 80V enhances the wear resistance of the coating,and no large coating peeling occurs,but the width of the wear zone is larger,about 0.18mm;When the negative bias voltage of the functional layer continues to increase to 120V,the coating adhesion and hardness are improved,the tool wear resistance is obviously improved,and the width of the wear zone is obviously reduced,about 0.14 mm;However,when the negative bias of functional layer increases to 150V,the negative bias sputtering of functional layer leads to the decrease of coating hardness and adhesion,and the decrease of coating wear resistance,and the width of the 5-hour wear band is approximately 0.18mm.

Fig.6 Comparison of wear patterns of tools worn for 5h.

4 Conclusion

AlCrN/TiSiN composite coating film with multi-layer structure was prepared by AIP and high-power pulsed magnetron sputtering coating process.The effects of negative bias of different functional layers on the surface,hardness,adhesion and cutting performance of the coating were investigated,and the following conclusions were drawn.

(1) With the increase of the negative bias of the functional layer,the number of droplets on the coating surface first decreases and then increases,and the droplets are the least and the roughness is the smallest under the negative bias of the functional layer of 120V;

(2) The hardness and adhesion of the coating are negatively biased in the functional layer.The maximum is 34.6GPa and 117.5 N at 120V,respectively.

(3) The negative bias coating stress of the lifting functional layer increases;

(4) When cutting 45 steel,the cutting performance of functional layer with negative bias of 120V is the best,and the service life is 8 hours.

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