销盘式高温高速摩擦磨损试验机的设计7-科技文原文.pdf

Surface and Coatings Technology 167 (2003) 5967 0257-8972/03/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972(02)00882-4 Corrosion resistance of ZrN films on AISI 304 stainless steel substrate Wen-Jun Chou, Ge-Ping Yu, Jia-Hong Huang* Department of Engineering and System Science, National Tsing Hua University, Hsinchu 300, Taiwan, ROC Received 28 June 2002; accepted in revised form 6 December 2002 Abstract The corrosion resistance of ion-plated Zr, ZrN and ZrNyZr films on commercial AISI 304 stainless steel has been investigated by electrochemical measurement. The electrolyte, 0.5 M H SOcontaining 0.05 M KSCN, was used for the potentiodynamic 24 polarization. The potentiodynamic scan was conducted from y800 to 800 mV (SCE) with scan rate ranging from 10 to 600 mVymin. The NyZr ratios of the ZrN films determined by X-ray photoelectron spectroscopy (XPS) were essentially stoichiometric. The composition depth profiles measured by secondary ion mass spectrometry (SIMS) indicated that the compositions in the ZrN films were uniform from the film surface to the 304 stainless steel substrate. Experimental results showed that the corrosion current density Iand passive current density I increased with increasing polarization scan rate for the bare AISI 304 stainless corrp steel specimens. Compared with the bare substrate, the Iand Ifor the coated specimens decreased at least 1 order of corrp magnitude. The bi-layer ZrNyZr coating possessed the highest corrosion resistance among the three coated-specimens. Because of the cathodic control of the galvanic corrosion, the corrosion potential of the coating specimens was slightly higher than that of bare metal substrate. The corrosion power Q, i.e. the integrated electric charge per unit area of the specimen during potentiodynamic polarization test, was an effective index to evaluate the corrosion resistance of the coated stainless steel substrate. The pinhole density played a significant role in corrosion resistance of the transition metal nitride coatings. Normalized critical passive current density (NI) was closely related to the exposure area, and a linear relationship between Q and NIwas held. critcrit ? 2002 Elsevier Science B.V. All rights reserved. Keywords:ZrN; Corrosion; Potentiodynamic polarization; Scan rate; Pinhole 1. Introduction The transition metal nitride coatings have some excel- lent properties, such as high hardness, good wear resis- tance,chemicalstability,corrosionresistanceand attractive colors, and therefore are widely used in indus- try in the current decade, especially TiN coating. More recently, ZrN started to attract more attention for its better corrosion resistance, comparable mechanical prop- erties and warmly golden color, compared to the corre- sponding properties of TiN film w14x. Vapor deposition methods including PVD and CVD are the popular techniques used to prepare transition metal nitride films. It is well known that PVD or CVD coatings normally have numerous inherent microscopic *Corresponding author. Present address: 101 Kuang Fu Road, Sec. 2,Hsinchu,Taiwan;Tel.:q886-35715131x4274;fax:q886- 35720724. E-mail address: .tw (J.-H. Huang). defects, e.g. pinholes or porosity w5x, which are detri- mental to a corrosion protective film. Localized corro- sion would initiate at these sites, when the corrosive medium penetrates through the pinhole and reaches the metal substrate underneath. Several ways for reducing the pinhole rate, pinhole area per unit coating area, have been proposed such as: increasing the coating thickness w68x; modifying the film structure from columnar to equiaxed w9x; controlling the bias potential during the film deposition w1012x; and multilayering w10,11,13,14x. Many techniques have been proposed to evaluate the corrosion resistance and pinhole rate of the coatings w6,7,11,1417x. The method of potentiodynamic polari- zation with varying scan rate was introduced in the present study. By changing the potentiodynamic polari- zation rate, one may obtain useful thermodynamic and kinetic information simultaneously. The pinhole rate can 60W.-J. Chou et al. / Surface and Coatings Technology 167 (2003) 5967 Table 1 Summary of electrochemical testing conditions and testing results MaterialScan rateEcorrIcorrIp (mVymin)wmV(SCE)x(mAycm ) 2 (mAycm ) 2 304SS10y44735.83.3 20y44539.017.4 50y44460.9101.9 100y44969.063.6 600y45495.21172.7 Zry304SS10y43610.62.5 20y44123.83.5 50y45112.94.9 100y43316.312.0 600y43411.026.7 ZrNy304SS10y4331.24.7 20y4384.13.7 50y4381.85.3 100y4353.76.7 600y4426.541.9 ZrNyZry304SS10y4371.80.6 20y4310.54.6 50y4321.216.7 100y4351.54.4 600y4371.317.6 Electrolyte: 0.5 M H SO containing 0.05 M KSCN was used for 24 the potentiodynamic polarization conducting from y800 to 800 mV. Fig. 1. The AFM topographic image of ZrN-coated specimen. The surface roughness of the specimen is 4.97 nm. be evaluated by critical passive current density w6,7x defined as: Ifilmy304SS. crit NIs crit I304SS. crit where NIis the normalized critical passive current crit density, I(filmy304SS) is the critical passive current crit density for the film-coated specimen; I(304SS) is the crit critical passive current density for the bare 304 stainless steel substrate. The aim of the present study is to investigate the corrosion resistance of ion-plated Zr, ZrN and ZrNyZr films on commercial AISI 304 stainless steel substrates. The pinhole rate is also examined with various poten- tiodynamic polarization tests on the coatings. 2. Experimental details 2.1. Materials A commercial AISI 304 mirror-like polished stainless steel was used as substrate material. The material had a composition of 0.1% Cu, 0.14% Co, 0.44% Si, 1.18% Mn, 8.37% Ni, 18.57% Cr and the balance being Fe. Prior to the coating process, the specimens were ultra- sonically cleaned in acetone and ethanol progressively, each for 5 min, and dried for approximately 20 min in a pre-vacuum dryer. The coating process was carried out in a hollow cathode discharge ion plating (HCD-IP) system, made by VMC-TIGOLD Co., Japan. The coating system and process have been described elsewhere w11,18x. The selection of the deposition conditions was based on the earlier research w18x where the microstructure and prop- erties of ZrN on Si (100) substrate were investigated thoroughly with respect to substrate bias ranging from floating to y300 V. The substrate bias at y50 V showed the optimum properties. In this study, the spec- imens were deposited at 350 8C, the HCD gun power was 6 kW, substrate bias was y50 V, and Ar and N2 61W.-J. Chou et al. / Surface and Coatings Technology 167 (2003) 5967 Fig. 2. The potentiodynamic polarization curves for: (a) bare 304 stainless steel; (b) Zry304SS; (c) ZrNy304SS; and (d) ZrNyZry304SS with various scan rate in solution 0.5 M H SO containing 0.05 M KSCN. 24 partial pressures were 0.133 and 0.0266 Pa, respectively. The durations for depositing Zr and ZrN were 36 and 40 min, respectively, which were chosen to control the film thickness for each layer to be 600 nm. For the bi- layer ZrNyZr specimens, the durations for depositing Zr and ZrN were reduced and the total coating thickness was controlled to be the same as single-layer specimens. The composition depth profiles of ZrN films were obtainedusingsecondaryionmassspectrometry (SIMS), and the thickness of both Zr and ZrN was determined from the SIMS profiles. The NyZr ratios were measured by X-ray photoelectron spectrometry (XPS). The surface morphology of coated specimens was observed by atomic force microscopy (AFM). 2.2. Electrochemical tests Prior to the electrochemical test, the specimens were ultrasonically cleaned in acetone. The potentiodynamic polarization tests were preformed on all specimens, including bare AISI 304 stainless steel, using an EG and (b) 600 mVymin. Fig. 4. The variation of corrosion current density with scan rate. Fig. 5. The passive current density changes with scan rate for all series of specimens. y800 to 800 mV (SCE). Various scan rates were introduced, which were 10, 20, 50, 100 and 600 mVy min. After each potentiodynamic polarization test, the cor- rosion potential, E, and the corrosion current density, corr I, can be determined by Tafel plot w20x. The critical corr passive current density, I, and the passive current crit density, I , can be obtained from the potentiodynamic p polarization curve. The corrosion power Q is defined as the integration of the current density by time within a certain range in the active region of the potentiodynamic polarization curve from y400 to y250 mV. Qs j dt a| where j is the active current density (Aycm ), t is the 2 a time (s), and Q is the corrosion power (Cycm ) w21x. 2 The corrosion power Q represents the extent of metal substrate dissolved into the electrolyte. Therefore, small- er value of corrosion power Q can be correlated to higher corrosion resistance of the specimens. The surface morphology of the specimens after potentiodynamic polarization test was observed using a scanning electron microscope (SEM). 3. Results 3.1. Characteristics of the deposited films The single layer ZrNy304SS and bi-layer ZrNyZry 304SS were all gold-colored, and the single layer Zry 304SS had the same color of Zr metal. Fig. 1 shows the 63W.-J. Chou et al. / Surface and Coatings Technology 167 (2003) 5967 Fig. 6. Corrosion potential vs. scan rate for all specimens. Fig. 7. (a) The critical passive current density and (b) normalized critical passive current density with respect to scan rates. AFM topographic image of ZrN-coated specimen. A smooth surface with roughness less than 5 nm was obtained. The film surfaces look compact and contain a few defects observed by the SEM micrographs. From the previous study w18x, both cross-sectional SEM and TEM images (figs. 1 and 6 in Chou et al. w18x) showed that the ZrN films have a densely columnar structure with an average column width of 50 nm. The NyZr ratios, determined from XPS measurements, are essen- tially stoichiometric (NyZrs1) for all ZrN films, which is consistent with our earlier research w18x. From the SIMS composition depth profiles, the film thickness of each coating can be determined as 600 nm. The com- positions are also uniformly distributed for each layer from the film surface to the 304 stainless steel substrate. The testing conditions of the specimens and results are summarized in Table 1. 3.2. Potentiodynamic polarization Fig. 2 shows the potentiodynamic polarization curves for bare 304 stainless steel, Zr, ZrN and ZrNyZr with various scan rates in solution 0.5 M H SO containing 24 0.05 M KSCN, respectively. Table 1 lists the values of Eand Ifor all series of specimens obtaining from corrcorr the Tafel plot and those of Iand I from the poten- cirtp tiodynamic polarization curve. The potentiodynamic polarization curves for a series of specimens are com- pared in Fig. 3(a,b) at two different scan rates, 100 and 600 mVymin. From Table 1 and Fig. 3, apparently, the corrosion current density decreases abruptly for coated specimens and the polarization curves can be divided into two groups: coated and uncoated specimens. Fig. 4 delineates that the bi-layer ZrNyZr coating, with the lowest I, exhibits the highest corrosion corr resistance among the three coatings. Compared with the bare 304 stainless steel, the coated specimens provide good corrosion resistance and the corrosion rate decreas- es at least an order of magnitude for every scan rate. Figs. 4 and 5 also show that both corrosion current density and passive current density increase with the increase of polarization rate for bare 304 stainless steel, which is consistent with the results in the recent studies w2224x. Mansfeld w25x pointed out that the trend for passive current density with different scan rates should be ignored, because this value is potential-dependent. However, the passive current density provides useful kinetic information. The stability of the passive film can be represented by the passive current density, i.e. larger I sless stable film. At higher scan rate, the anodic p reaction is increased and the film formed is less protec- tive; consequently larger corrosion current density and passive current density are expected w26x. Fig. 5 depicts the passive current density varying with scan rates for all series of specimens. It can be seen that in general 64W.-J. Chou et al. / Surface and Coatings Technology 167 (2003) 5967 Table 2 Summary of corrosion current density Iand corrosion power Q crit MaterialScan rateIcritNIcritQNQ (mVymin)(mAycm ) 2 (Coulombycm ) 2 304SS10198.158.13 20167.765.11 50138.062.26 100144.851.14 600127.750.16 Zry304SS10191.820.972.900.36 20195.541.172.030.40 5022.890.170.350.16 10036.02 60016.300.130.020.12 ZrNy304SS1019.770.100.500.06 2010.260.060.340.07 503.060.020.060.03 1003.850.030.040.04 6003.600.030.010.04 ZrNyZry304SS1029.350.150.340.04 205.030.030.050.01 503.350.020.040.02 1002.380.020.020.02 6001.270.010.0010.01 the passive current density increases with scan rate; especially for the bare stainless steel specimens, the passive current densities show an increase of more than 2 orders of magnitude as the scan rate increases from 10 to 600 mVymin. On the other hand, for the coated specimens the increase of passive current density is less distinct and within one order of magnitude. Fig. 6 shows the results of corrosion potential with respect to scan rate for all specimens. Similar to the corrosion current density and passive current density, the Evalues are obviously divided into two groups, corr coated and uncoated. The bare 304 stainless steel (approx. y450 mV) shows slightly lower corrosion potential than other coated specimens (approx. y435 mV). According to the precision tests, the deviation of the corrosion potential and corrosion current density in EG on the contrary, the 304 stainless steel substrate has a lower corrosion potential (y450 mV) and the minor exposure area through the pinhole. This combination fulfills the basic requirement of the typical cathodic control of galvanic corrosion. Therefore, the coated specimens have a slightly higher corrosion potential than the bare metal substrate. The evidence of pitting corrosion can further support the above argument. It is clear that from the polarization curves shown in Fig. 2, the passive current is unstable for the coated specimens. The scatter of the current density in the passive region can be explained by the clogging of the corrosion product in the pinholes w3x and the breaking down of the films on coated specimens. The present study shows that the bi-layer or multi- player coating may enhance the corrosion resistance of the specimens. In Figs. 4 and 5, compared with the bare substrate, the Iand Idecreases at least 1 order of corrp magnitude for the bi-layer specimens, and the bi-layer ZrNyZr coating exhibits the highest corrosion resistance of all coated specimens. Since corrosion occurs via the diffusion of the electrolyte through pinholes and attack- ing the underneath substrate, the pinhole number and size are closely related to the corrosion resistance of specimens. The index NIis introduced to represent crit the exposure area of the coating. In Fig. 9, the ZrNyZr bi-layer coating shows the lowest NIvalue for all crit scan rates. The corrosion power Q increases with the increase of the exposure area of coatings, NI, as crit shown in Fig. 8(b), which further supports the pinhole corrosion mechanism. It also suggests that bi-layer coating may interrupt the pinhole connection through the coating surface to the underneath of metal substrate, minimize the exposure area, and thereby improved the corrosion resistance. The changing scan rate in this study was designed for revealing the kinetic dependence of corrosion parame- ters. As mentioned earlier, the increase of scan rate leads to the increase of anodic reaction, and the oxide film formed on the metal surface is less protective. Therefore, the trends of Iand I with respect to the corrp scan rate, as shown in Figs. 4 and 5, respectively, can provide kinetic information for both coated and bare 304 stainless steel specimens. Fig. 4 demonstrates that Iincreases with scan rate for bare 304 stainless steel corr but not as obvious in the other three coated specimens. Since the exposure area of the substrate metal in the coated specimens is much smaller than that in bare metal specimen, the effect of increasing anodic reaction is not as effective as that for bare 304 stainless steel. In addition, since the pinhole rate is different in the three coatings, the corrosion current densities are different. On the other hand, I in Fig. 5 shows the stability of p the oxide film, and therefore the oxide film stability between the coated specimens is not significant. Again, due to the small exposure area of the substrate metal, the effect of changing scan rate on the oxide film stability in coated specimens is much less than that on bare 304 stainless steel specimens. 5. Conclusion The corrosion resistance of Zr, ZrN and ZrNyZr films deposited on 304 stainless steel was investigated using potentiodynamic polarization tests with varying scan rates. The bi-layer ZrNyZr coating exhibits the highest corrosion resistance compared to the other two single layer coatings. The corrosion power Q, the integrated electric charge per unit area of the specimen during potentiodynamic polarization test, is an effective index to evaluate the corrosion resistance of film-coated spec- imens. Furthermore, the corrosion power Q increases with increasing NI. crit Acknowledgments This research was funded by the National Science Council of the Republic of China under the contracts NSC-89-2216-E-007-036 and NSC-89-27457-007-002- NU. The authors also appreciate the instrumental support of the Instrumentation Center at National Tsing Hua University, Taiwan, ROC. References w1xE.