Effect of NbN and ZrN ?lms formed by magnetron sputtering on Ti and porcelain bonding
Shu-Shu Wang a ,Yang Xia a ,La-Bao Zhang b ,Han-Bing Guang c ,Ming Shen a ,Fei-Min Zhang a ,?
a Institute of Stomatology,Nanjing Medical University,Nanjing 210029,China
b Research Institute of Super Conductor Electronics,Nanjing University,Nanjing 210093,China c
Denture Machining Center of Stomatology Hospital of JiangSu Province,Nanjing 210029,China
a b s t r a c t
a r t i c l e i n f o Article history:
Received 17April 2010
Accepted in revised form 16August 2010Available online 24August 2010Keywords:
Niobium nitride (NbN)Zirconium nitride (ZrN)Magnetron sputtering Titanium Porcelain Bonding
Niobium nitride (NbN)and zirconium nitride (ZrN)were deposited on Ti substrates by direct-current (DC)reactive magnetron sputtering;the deposited NbN and ZrN ?lms served as intermediate layers of a Ti and porcelain interface.X-ray diffraction (XRD)results proved that the deposited NbN and ZrN ?lms were polycrystalline with a cubic microstructure.The Ti and porcelain bonding strength of the samples in Group Control (27.2±0.75MPa),Group NbN (43.1±0.59MPa),and Group ZrN (52.4±0.80MPa)were measured.The surface roughness in the case of Group Control (1.863±0.10μm),Group NbN (2.343±0.07μm),and Group ZrN (2.346±0.10μm)was also investigated.Statistical analysis showed that both ?lms helped improve the Ti and porcelain bonding strength and increase the surface roughness.Scanning electron microscopy (SEM)results showed that no apparent oxide layer was formed at the Ti and porcelain interface in both Group NbN and Group ZrN.Energy-dispersive X-ray spectroscopy (EDS)results showed that ZrN was more effective in preventing Ti oxidation than was NbN.Overall,the experimental results showed that the deposition of both NbN and ZrN ?lms helps improve the Ti and porcelain bonding strength and that ZrN ?lms are more effective.
Crown Copyright ?2010Published by Elsevier B.V.All rights reserved.
1.Introduction
Ti and its alloys have good biocompatibility,corrosion resistance,and desirable physical and mechanical properties,and further,they are inexpensive;hence,they are being increasingly used in dental implants and prostheses [1–5].Researchers believe that Ti and ceramic restoration will gain importance in dental applications [6].However,Ti reacts readily with oxygen at high temperatures [7].The highly oxidative character of the Ti surface and the αto βtransformation of Ti at 882°C could be the possible reasons for the weak bonding between porcelain and Ti [8–10].Some manufacturers have introduced low-fusing porcelains that have favorable thermal expansion coef ?cients and can bond to Ti at low-fusing temperatures (b 850°C)[11].However,these low-fusing porcelains do not effectively prevent Ti oxidation,and therefore,the weak Ti and porcelain bonding is a major problem encountered when using Ti for dental restorations.One suggested solution for preventing Ti oxidation is
the deposition of an intermediate layer onto the Ti surface prior to porcelain bonding [12].
In the past decade,many transition metal nitride ?lms such as zirconium nitride (ZrN)?lms have become commercially important because of their high hardness,high thermal and chemical stability,and appealing aesthetic color [13,14].Niobium nitrides (NbN)have also become increasingly popular because of their high hardness [15–17],remarkable corrosion resistance [18],and excellent chemical stability at high temperatures (1000°C)[19].Many researchers have studied the conditions required for NbN and ZrN ?lm deposition by magnetron sputtering and analyzed the properties of the deposited ?lms [20,21].However,very few researchers have investigated the use of NbN and ZrN ?lms for dental applications.Because of their high thermal and chemical stability,ZrN and NbN ?lms have been used as intermediate layers between Ti and porcelain in this study.Direct-current (DC)reactive magnetron sputtering has been used to deposit NbN and ZrN ?lms.The aim of this research is to investigate the oxidation of Ti substrates and stability of the NbN and ZrN ?lms under porcelain sintering conditions and to identify the in ?uence of the deposition of these two ?lms on Ti and porcelain bonding.
2.Experimental procedures
Commercially pure cast Ti substrates (25.0mm×3.0mm×0.5mm)were selected as the test samples.Surface treatment was carried out by
Surface &Coatings Technology 205(2010)1886–1891
?Corresponding author.Institute of Stomatology,Nanjing Medical University,136Han Zhong Road,Nanjing 210029,China.Tel.:+86138********;fax:+862586516414.
E-mail addresses:suewang198579@https://www.doczj.com/doc/7018571534.html, (S.-S.Wang),
xiayangxy@https://www.doczj.com/doc/7018571534.html, (Y.Xia),zhanglabao@https://www.doczj.com/doc/7018571534.html, (L.-B.Zhang),guang662@https://www.doczj.com/doc/7018571534.html, (H.-B.Guang),mingshen85@https://www.doczj.com/doc/7018571534.html, (M.Shen),fmzhang@https://www.doczj.com/doc/7018571534.html, (F.-M.
Zhang).
0257-8972/$–see front matter.Crown Copyright ?2010Published by Elsevier B.V.All rights reserved.doi:
10.1016/j.surfcoat.2010.08.066
Contents lists available at ScienceDirect
Surface &Coatings Technology
j o u r n a l h om e p a g e :w w w.e l s ev i e r.c o m /l o c a t e /s u r fc o a t
sandblasting and sputtering.Sandblasting is regularly used for treating Ti surfaces prior to porcelain sintering.The surfaces of the Ti substrates were blasted with120μm Al2O3particles under0.2MPa pressure at a45°angle.The source of Al2O3particles was10mm away from the Ti substrates.After sandblasting,NbN and ZrN were sputtered onto the surface of the Ti substrates by DC reactive magnetron sputtering in an Ar/ N2gas mixture.The Ar:N2ratio was6:1.Nb and Zr discs(purity:99.9%; diameter:73.0mm;thickness:3.0mm)were used as the targets.The substrate-to-target distance was60.0mm.The vacuum chamber was evacuated to a base pressure of2×10?5Pa.Before deposition,the Ti substrates were cleaned by bombardment with Ar ions.Both?lms were deposited under the following conditions:target power,350W;working pressure,0.3Pa;and deposition time,15min.Deposition was carried out at room temperature under cooling by water circulation.
A total of45Ti substrates were randomly chosen and divided into three groups(n=15):Group Control,sandblasting only;Group NbN, sputtering NbN?lms after sandblasting;and Group ZrN,sputtering ZrN ?lms after sandblasting.Each group was processed under different surface treatments as described above.After the treatments,the substrates were subjected to steam and ultrasonic cleaning.
Low-fusing porcelain was brushed onto the treated surfaces of the Ti substrates.A bonding agent(thickness:0.2mm),an opaque layer (thickness:0.2mm),and a dentin coat(thickness:0.6mm)were deposited onto the central region of the Ti substrates(8.0mm×3.0mm).The substrates were then sintered in a porcelain furnace by following the sintering schedule[12].The highest sintering temperature is 800°.
X-ray diffraction(XRD,D/max2500/PC,Rigaku Corporation,Japan), scanning electron microscopy/energy-dispersive X-ray analysis(SEM/ EDS,SEM-6300,Jeol Ltd,Japan),and digital microscopy analysis(Keyence Digital Microscopes VHX-600,Keyence Corporation,Japan)were used to study the samples.A three-point bending test was performed according to the ISO96931999[22]procedure for determining the Ti and porcelain bonding strength;the schematic diagram are displayed in Fig.1.The bonding strength(τ)was calculated from the following equation,which was obtained from ISO9693:30[22]:
τ=k×F fail N=mm2
where F fail is the maximum force(N)applied before debonding(also called failure load),and k is a constant(mm?2)determined from a graph in ISO9693[22].The value of k depends on the thickness and elastic modulus of the metal substrate used.For pure Ti,k was determined to be4.7mm?2.Ten porcelain sintering samples were tested for each experimental group.The surface roughness was measured by a handheld roughness-measuring device(TR200,Beijing Times Corporation,China).The mean values of the bonding strength and surface roughness were compared by analysis of variance (ANOVA)and Tukey's multiple comparisons to determine the statistical signi?cance of the mean differences amongst the groups. The statistical signi?cance was set at the0.05probability level.
3.Results and discussion
3.1.NbN and ZrN phases
The deposited NbN and ZrN?lms were silver and light yellow in color, respectively,with metallic highlights.There were no visible defects in either?lm.The phases in the deposited?lms are displayed in Fig.2.The diffraction peaks of Ti,NbN,and ZrN proved that the as-deposited,NbN, and ZrN?lms were polycrystalline with cubic microstructures,having (111),(200),(220),(311),and(222)orientations.The XRD spectra also showed peaks corresponding to the(100),(002),(101),(102),(110), (103),(112),and(201)orientations of Ti.In the spectra of the samples in the experimental groups,some of the Ti peaks overlapped with the nitride peaks.Different N2/Ar gas ratios resulted in the formation of different phases in the deposits.It was found that cubic NbN was formed at a low N2/Ar gas ratio.With an increase in the N2/Ar gas ratio,however,both cubic NbN and hexagonal NbN were formed[20].Despite their good chemical stability at high temperatures(1000°C)[19],NbN?lms have not been used in metal and ceramic dental restorations.Previously,we proved that ZrN helps in increasing the strength of Ti and porcelain bonding[23]. Singer et al.[24]discovered that the N2partial pressure affects the NbN microstructure.Further,N2partial pressure is a key parameter that determines the nature of the ZrN phases[25].Differences in the chemical composition of the NbN and ZrN?lms may lead to differences in the effect of?lm deposition on the Ti and porcelain bonding strength.According to Yotsuya et al.[26],a stable phase of stoichiometric ZrN x(x=1)should form when sputtering ZrN?lms.In this study,both NbN and ZrN?lms were deposited under the same sputtering conditions in order to achieve the same coating thickness,about1μm according to the deposition rate (0.1nm/s),so as to ensure a fair comparison of the in?uence of depositing these two?lms on Ti and porcelain bonding.
3.2.Surface roughness and bonding strength
The surface characteristics are often focused on because they can affect the adhesion force and various properties of the?lms[27].For surface roughness(Ra)measurements,we selected?ve8mm×3mm regions on the surface of each Ti substrate,which had an area of0.8mm×5mm.The results are shown in Fig.3.The Ra value of the polished Ti surface was 0.129±0.01μm.Group NbN(2.343±0.07μm)and Group ZrN(2.346±0.10μm)showed higher Ra values than did Group Control(1.863±0.10μm).The results of ANOVA proved that the deposition of NbN
and Fig.1.Schematic diagram of porcelain application and three-point bending
test.Fig.2.XRD results obtained for NbN and ZrN.
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ZrN ?lms helped in increasing the Ra value of the Ti substrates (P b 0.05).However,Tukey's comparisons did not reveal any statistical difference between Ra values of the two ?lm groups.Thus,according to these results,the Ra values of the sputtered NbN and ZrN ?lms were similar.
The results of the three-point bending test are also shown in Fig.3.A total of 10post-sintering samples were tested from each group.The bonding strength of the Group Control reached 25MPa,which is the minimum debonding/crack-initiation strength set by ISO 96931999standards for porcelain/metal combinations [22].The ZrN-sputtered specimens had the highest bonding strength.The statistical differences between the three groups were recorded by ANOVA (P b 0.05).These results con ?rmed that sputtering of Ti with either NbN or ZrN helped improve the Ti and porcelain bonding strength and that the use of these ?lms resulted in better adherence of porcelain to the Ti substrate surface.Tukey's multiple comparisons showed that statistical differences exist between the two experimental groups and that better Ti and porcelain bonding strength is achieved when using ZrN than when using NbN.
High surface roughness facilitates ef ?cient mechanical interlocking between Ti and porcelain [28].It is known that differences in Ra,which result from differences in the substrate treatment,may alter the Ti and porcelain bonding strength.Derand and Hero [29]observed that the strength of Ti and ceramic bonding is improved to a greater extent when using 250μm Al 2O 3particles than when using 50μm Al 2O 3particles.In this study,the increased Ra resulting from the use of NbN and ZrN might help increase the Ti and porcelain bonding strength.Airborne-particle abrasion helps improve the bonding strength by removing the loosely attached furrows,overlaps,and metal ?akes formed during the grinding process and thus affords better mechanical interlocking and increased surface area and wettability [30–33].In this study,because of the use
of
Fig.3.Surface roughness and bonding strength of Group Control,Group NbN and Group
ZrN.
Fig.4.SEM images and EDS results of Ti and porcelain interface.Panels a,b,and c show the interface image for Group Control,Group NbN,and Group ZrN.Panels a-EDS,b-EDS,and c-EDS show the EDS results for Group Control,Group NbN,and Group ZrN.NbN and ZrN regions are indicated by white arrows,while the region in which oxidation occurs is marked by black arrows,according to the EDS results.
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NbN and ZrN?lms,the Ti and porcelain bonding strength in the experimental groups becomes signi?cantly higher than that in the case of Group Control,thus con?rming the improved adherence of the ceramic to the coated metal.However,Ra does not appear to be the only factor that determines the bonding strength because the two groups(Group ZrN and Group NbN)show different bonding strengths,even though Ra is similar in both cases.The results obtained when using ZrN are better than those obtained with NbN,and the bonding strength is signi?cantly higher in the former case.These results indicate that these two?lms must also alter the characteristics of the Ti surface,apart from increasing Ra.
3.3.Interface analysis
SEM micrographs and EDS results of the cross-sectioned post-sintering samples before the three-point bending test are shown in Fig.4.No apparent oxide layer was observed at the Ti and porcelain interface in any of the groups.Fig.4(a)shows the interface image for Group Control.Several regions with precracks were observed in this image.Fig.4(a-EDS)shows the content of oxygen increased while that of Ti reduced at the same location on the Ti substrate,as indicated by arrows.
A de?nite increase in the oxygen content was also observed at the Ti and porcelain interface.These results proved that oxidation occurs both at the Ti and porcelain interface and in the Ti substrate.Fig.4(b)and(c)shows that the ZrN and NbN?lms helped improve the bonding between Ti and https://www.doczj.com/doc/7018571534.html,parison of the EDS results of Ti and porcelain interface revealed that the oxygen atoms penetrate the Ti surface to a greater depth in the case of Group Control than in the case of the other two groups. From Fig.4(c-EDS),it was apparent that no notable oxidation occurred at the Ti and porcelain interface in the case of Group ZrN.This con?rmed that both NbN and ZrN can prevent the penetration of the Ti substrates
by
Fig.5.Surface morphology of Ti substrates before sintering.Panels a,b,and c show the
surface morphology in the case of Group Control,Group NbN,and Group ZrN,
respectively.
Fig.6.Surface morphology of Ti substrates after sintering.Panels a,b,and c show the
surface morphology of Group Control,Group NbN,and Group ZrN,respectively.Minor
gaps are marked by white arrows.
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oxygen and that the use of these?lms helps prevent the formation of nonadherent oxides on the Ti surface.The oxygen content near the interface was found to be relatively lower when using the ZrN?lm than when using the NbN?lm.The bonding strength value of Group ZrN was higher than that of NbN,and hence,it is revealed that oxidizing reaction was effectively inhibited by ZrN?lms.Both?lms effectively prevented the Ti substrate from oxidizing.These results proved that Ti and porcelain had the best combination when the Ti surface was coated with ZrN,as con?rmed by the results of bonding strength measurements.It has been reported that the thickness of the ZrN?lm shows no signi?cant change even after annealing at500°C for1h in a vacuum furnace,implying that no?lm volume increase occurs[34].However,in some studies,it has been stated that the failure temperature of the barrier performance of extremely thin ZrN?lms is just above500°C[35,36].G.L.N.Reddy,et al.
[37]observed that the morphology of a ZrN?lm changes owing to blister formation when the?lm is annealed in static air for8h or longer.The highest?ring temperature is about800°C,and the?ring time is much longer than1h.However,in this study,no apparent?lm cracking or porcelain fracture was found at the Ti and porcelain interfaces in the two experimental groups.When ZrN?lms are exposed to air,the N atoms are gradually replaced by O atoms to form ZrO2[38].Oxygen has been reported to react preferentially with Zr rather than with Ti at high temperatures,e.g.,at800°C[39].This prevents oxidation of the Ti surface during sintering,allowing the formation of a chemical bond between the ?lm and the porcelain surface.There are very few reports on the reactivity of NbN?lms toward oxygen during porcelain sintering.The EDS results obtained in this study showed that NbN could also inhibit the oxidation at the interface and in deeper Ti substrates,compared with Group Control.It has been reported that Nb shows relatively higher reactivity toward oxygen and is stable as an oxide at600to800°C[40],which is close to the porcelain sintering temperature.We speculate that these oxides block oxygen penetration and protect the Ti substrate from oxidation.We could conclude from the EDS results that the N atoms of ZrN and NbN diffuse into the Ti substrates and that Nb and Zr atoms remain at the same location where they were deposited.
3.4.Stability of NbN and ZrN during sintering
For a more in-depth study of the behavior of ZrN and NbN during porcelain sintering,we placed the deposited samples without applying any bonding agent or porcelain layer(one from each group)inside a dental furnace and carried out porcelain sintering thermocycles (stimulated sintering).The surface morphology before and after sintering is shown in Figs.5and6,respectively.As could be seen from the SEM images,the Al2O3particles were amorphous and disorderly after sintering.No cracks were observed in the nitride?lms before sintering, whereas minor gaps(shown by white arrows in Fig.6)appeared after the sintering procedure.EDS measurements were performed to determine the elemental composition of the surface.In Group NbN,the oxygen content increased by9.14%and the Nb content decreased by7.99%after sintering.In Group ZrN,the oxygen content increased by10.55%and the Zr content decreased by6.35%after sintering.NbN has been reported to have good chemical stability at high temperatures(1000°C)[19].On the other hand,there are different views on the stability of ZrN at such high temperatures.In the present research,certain reactions that caused the reduction of these two nitrides were found to occur.XRD measurements were performed to analyze the possible products of this reduction (Fig.7).The XRD results did not reveal the formation of Nb oxide, con?rming that NbN is highly stable at the sintering temperature employed in this study.The XRD results,however,showed the
presence Fig.7.XRD results obtained for NbN and ZrN after stimulated
sintering.
Fig.8.Fractured surfaces of Ti substrates.Panels a,b,and c show the fractured surface in
the case of Group Control,Group NbN,and Group ZrN,respectively.
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of ZrO 2.The ZrO 2peaks were much smaller than the ZrN or Ti peaks,indicating that only a small amount of the oxides existed on the sintered surface.According to the XRD results,no Ti oxidation occurred.This con ?rmed that oxygen atoms in ?ltrated into the ZrN coating but did not reach the Ti substrates.Both ZrN and NbN could prevent Ti oxidation under the present sintering conditions,regardless of whether they were stable or converted into oxides.We hypothesize that the increased oxygen content at the surface in the case of Group NbN,as shown by the EDS results,is due to air-adsorbent oxygen.Further studies are required to con ?rm this hypothesis [41].
3.5.Observations of fractured surface of post-sintering samples The fractured surfaces of the post-sintering samples after the three-point bending test are shown in Fig.8.An EDS test proved that the dark areas corresponded to the residual porcelain,while the gray ones were interlayer or exposed Ti surfaces,respectively.Only points of porcelain were observed in the case of Group https://www.doczj.com/doc/7018571534.html,rger areas of porcelain were distributed randomly on the Ti surface in the case of the samples treated with NbN or ZrN.
Three dimensional (3D)images of the fractured Ti surface in the Group NbN and Group ZrN samples were recorded by using a digital microscope (Fig.9).The remaining porcelain (marked by white arrows in Fig.9)was partly embedded in the Ti substrate.The images showed the presence of fractures inside the porcelain body.The wettability between Ti and porcelain was improved,because of which better mechanical interlocking was achieved.These results indicated that the Ti and porcelain bonding strength increased when the Ti surface was coated with both NbN and ZrN ?lms.4.Conclusions
NbN and ZrN ?lms fabricated in this study were polycrystalline with a cubic microstructure.The results of a three-point bending test
indicated that both these nitride ?lms could help increase the Ti and porcelain bonding strength.The bonding strength was the highest when ZrN was used.The combination of Ti and porcelain in the case of Group NbN and Group ZrN was better than that in Group Control.EDS analysis of the Ti and porcelain interface showed that both the aforementioned ?lms could effectively prevent the adverse oxidation of Ti substrates.Thus,we concluded that both NbN and ZrN ?lms prevent Ti oxidation and that they can be used as intermediate layers to improve the Ti and porcelain bonding strength.Although ZrN was oxidized to ZrO 2under stimulated sintering,it was better at blocking the penetration of O 2than was NbN.This was the possible reason for the relatively high bonding strengths of the ZrN-coated Ti surfaces.Acknowledgements
This work was supported by grants from the Jiangsu Province International Technology Cooperation Plan (BZ2008059)and the Jiangsu Province Natural Foundation (BK2009137).The authors would like to thank the Analysis and Testing Center of Nanjing University and Nanjing Normal University for their help,and also express our thanks to Dr Karl Koerner for his help.References
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