II. Functional Coating and Surface Modification of Materials
Transition metal nitride coating has been widely used as protective coatings because of its high hardness, good wear and corrosion resistance. Recently, two kinds of nanostructured ceramic materials have been intensely studied to modify the properties of binary coatings to overcome environmental demands.
One is to add other elements into binary coatings to form ternary films, such as TiAlN and TiSiN. TiN with addition of Al can enhance the mechanical strength, caused by the solution hardening. Veprek proposed the strengthening mechanism of nano-composite that the nano-sized TiN grain was surrounded by the amorphous SiNx matrix in the Ti-Si-N coating based on the three dimensional nanostructure. In additional to high hardness, the ternary nitride systems, incorporating the Al and Si, revealed excellent oxidation and corrosion resistance.
The other way is to deposit two kinds of materials alternately, such as metal/metal, metal/nitride, and nitride/nitride stacks, which can provide superior mechanical and chemical properties to single-layered coatings by the specific interface. For mechanical applications, the nitride/nitride multilayers, including multilayered TiN/CrN, CrN/WN, and TiN/NbN coatings, are the most common candidates owing to their high hardness, chemical inertness and toughness. Barshilia et al. reported that the hardness of CrN/TiAlN coatings was strengthened higher than 30 GPa. Besides, the oxidation temperature and corrosion resistance of multilayer coating was also improved, as compared to the single-layered CrN coating.
1. Currently on-going studies
1.1 Tribological behavior of CrN/WN multilayer coatings
CrN monolayer coating and CrN/WN multilayer coatings were deposited on the silicon (100) substrate by ion-beam assisted deposition process. The bilayer period of these coatings was controlled at 8nm and 30nm respectively. The cross-sectional morphology of nanoscaled multilayer coatings was characterized by scanning electron microscopy and transmission electron microscopy. The wear resistance of CrN/WN multilayer coatings and CrN monolayer coating was investigated using a pin-on-disc tribometer. The surface roughness (Ra) of the coatings was evaluated by atomic force microscopy, and that of CrN and WN monolayer coating was 6.7 and 5.9 nm, respectively. The employment of multilayer configuration in CrN/WN coating with bilayer period of 8 nm and 30 nm effectively reduced the surface roughness down to 1.9 and 2.2 nm, respecctively. The friction coefficient of CrN monolayer coating and CrN/WN multilayer film with a bilayer period of 30 nm was 0.63 and 0.31, respectively. Owing to the high hardness/elastic modulus ratio, as well as the dense structure and the smooth surface roughness, the CrN/WN multilayer coatings exhibited better wear resistance in the consideration of friction coefficient and the worn surface morphology.
1.2 Effect of heat treatment on mechanical properties and microstructure of CrN/AlN multilayer coatings
Polycrystalline CrN/AlN multilayer coatings were deposited by RE magnetron sputtering on silicon (001) substrates. The bilayer periods of CrN/AlN were controlled from 4 nm to 20 nm by the use of shutters, which were adjusted by a programmable logic control (PLC). To evaluate the thermal stability, the films were annealed at 500, 600, 700, 800, and 850 oC, for I h in both vacuum and air environments. The phase transformation during thermal evolution was studied by X-ray diffraction (XRD). The microstructure of CrN/AlN multilayer coatings as-deposited and after annealing was observed by transmission electron microscopy (TEM). The hardness of as-deposited CrN/AlN coating with a period of 4 nm was 28.2 GPa, which was 60% higher than that predicted by the rule of mixtures. The hardness of CrN/AlN multilayer coatings annealed at 850 oC in vacuum remained similar to the as-deposited state, and the nano-layered structure still persisted. The thermal stability of CrN/AlN coatings was better than that of CrN coating. The hardness degradation ratio of CrN/AlN coating with modulation period of 4 nm was only 8.1% at 700 oC, which was superior to that of a simple CrN coating.
1.3 Tribological behavior of CrAlSiN/W2N multilayer coatings deposited by DC magnetron sputtering
The CrAlSiN/W2N multilayer coatings were fabricated by DC magnetron sputtering. The bilayer periods of multilayer films were controlled in the range from 3 to 20 nm. The cross-sectional structure of multilayer and monolayer coatings was evaluated by transmission electron microscopy (TEM). The wear behavior of monolayer and multilayer coatings was investigated by a pin-on-disc tribometer. The nano-scratch tester was employed to study the crack propagation of scratched coatings. The images of wear scars were observed by optical microscopy (OM). The cross-sectional image of scratched films was analyzed by transmission electron microscopy (TEM). Owing to the nano-layered structure and higher hardness (or H/E ratio), the multilayer coatings exhibited better wear resistance than homogeneous films. The coefficient of friction of CrAlSiN/W2N multilayer coating with a bilayer period of 8 nm was around 0.6, and that of CrAlSiN homogeneous film was about 0.8. Different crack propagation mechanisms of CrAlSiN/W2N multilayer and CrAlSiN monolayer coatings were proposed and discussed.
1.4 The effects of silicon content on the corrosion behavior of CrAlSiN coatings
The amorphization was achieved by adjusting the Si content of the coating. The nanocomposite structure of the modified CrAlSiN coating was investigated by TEM techniques, and the grain refinement by adding Si was verified. It was found that the columnar structure of CrAlN coating could be altered to a compact one after Si addition, and the polarization resistance of the coated sample was thus improved. Therefore, the CrAlSiN coating with high hardness, good thermal stability and corrosion resistance was suggested as a potential candidate coating for tool steels.
1.5 Microstructure characterization and corrosion properties of nano-multilayered CrAlN/SiNx (or TiAlN/SiNx) coatings
Both CrAlN (or TiAlN) and SiNx layers were deposited periodically by radio frequency reactive magnetron sputtering. In the multilayered CrAlN/SiNx (or TiAlN/SiNx) coatings, the thickness of CrAlN layer was fixed to 4 nm, while that of SiNx layer was adjusted from 4 to 0.3 nm. The dependence of the thickness of SiNx layer on the preferred orientation, crystalline behavior, microstructure and mechanical properties of the multilayered coatings were discussed with the aid of XRD patterns, SEM and HRTEM. The corrosion resistance of the multilayered CrAlN/SiNx (or TiAlN/SiNx) coatings in 3.5 wt.% NaCl solution was also investigated by the Tafel measurement and electrochemical impedance spectroscopy (EIS).
Amorphous SiNx layer was evidently transformed to a crystallized one, when its thickness was decreased from 1 to 0.3 nm. The crystalline SiNx layer grew epitaxially and formed the coherent interface with the CrAlN layer, enhancing the hardness of the multilayered CrAlN/SiNx coating to 33 GPa. In contrast, with further increasing the thickness of SiNx layer to 1 nm, the SiNx was transformed to an amorphous structure, destroying the coherent interfaces, and decreasing hardness of the multilayered CrAlN/SiNx coating.
It was also revealed that when the structure of SiNx layer was transformed from crystalline to amorphous, the multilayered films were changed from a columnar microstructure to a dense one. The denser microstructure exhibited a lower corrosion current density and a higher corrosion impedance, indicating that the obtained multilayered coatings had better corrosion protection.
1.6 Study on the development of non-sticking coating on IC test interposers
ICs test can be categorized into two major sections: CP (chip probing) and FT (final test). CP acts to distinguish damaged or failed BITS from each single die on a bare wafer, while FT is to find connection failures between Au wires and open-short out from each single assembled IC. In either CP or FT test, a probe card or an interposer is required to connect tested ICs with testers to mark substandard dies with prints in CP and single out disqualified ICs from qualified ones in FT.
In FT test, the most commonly used interposer called pogo pin is typically made of Be-Cu alloy with a 3-5 μm Ni coating and a 1-3 μm Au layers outside in sequence. Since Au naturally exhibits good wettability with Sn, the tested pogo pins would unavoidably stick with the contacted solder balls and leads of ICs after a long period of test for several thousands of repeated contacts, leading to a substantial test yield loss due to the increase of contact resistance.
This study aims to replace the traditionally combined Ni with Au layers deposited outside the Be-Cu base metal of pogo pins with a 20Ni-80Pd layer by the barrel electroplating process. The selected 20Ni-80Pd alloy was proven to be a promising candidate for a non-stinking coating development on pogo pins for its wear resistance is as good as Ni’s and contact resistance is as low as Au’s. Furthermore, unlike Au, the 20Ni-80Pd alloy with relatively inferior wettability to Sn can reduce the effect of Sn sticking on pogo pins, while maintain a low contact resistance during test. By precisely controlling such electroplating parameters as magnitude of current, time, rotational speed of chemical solution, and concentration of chemical solution, microstructures of the plated 20Ni-80Pd layer to exhibit better non-sticking capability against Sn can be derived. Finally, a test house (MXIC) will assist this study in testifying the 20Ni-80Pd-based pogo pins with similar testers for practical ICs tests to verify their effectiveness on Sn sticking resistance during test.
1.7 Study on the development of smart materials with self-repairing capability
The concept of self-repairing (self-healing) materials coming from a cut wound people suffer from on skin would heal itself along with the time increase as the wounded cells are replaced by healthy ones to get recovery. Similarly, if any substance, particularly for nano-capsules, containing healing agent can be buried or implanted in advance into materials that may happen corrosion, rifts or crafts from being attacked by force or chemical. It is assumed that those wounded parts will temporarily heal themselves by releasing the inner healing agent to prevent the said defects from becoming worse to extend materials life.
The newly proposed method can be utilized to reduce the extent of damage of the PCB’s pads, typically requiring contacting IC interposers in test as many as millions of times with large force. In bio-medical application, it might possibly be used to fully encompass the surface of a stent, a delicate hollow metal bracket for heart disease patents to make their clogged blood vessels widen to prevent vascular thrombosis. Furthermore, it’s also highly expected to be coated on the surface of an aircraft to increase flight safety by avoiding rifts or cracks occurring due to metal exhaustion, which commonly happen for a metal in an environment where pressure changes frequently. Materials with the self-repairing capability are considered as “smart materials”
Two different mechanisms for materials self-repairing are proposed. One is the controlled agents (adhesives) required to be released manually or automatically from the implanted matrix to reach materials repair by making the rifts or crafts on the material surfaces fully filled out. This method aims for electrical products repair, e.g., PCB boards or IC interposers. The other is the agents delivered to the right place at right time as their surrounded environments are stimulated with the change of light, heat or pressure. The implanted nano-capsules can release the agents only when kinetic energy (KE) exists in the system or environment to break out their cores. It is potential for bio-medical applications where usually demand to exactly control agents release at the time doctors desire.
2. Previous Research Issues
2.1.Nitride Coating
TiN coating, which exhibits the characteristics of high hardness and gold-yellow color, draws enormous attentions and has been widely used in versatile industrial application. The mechanical properties and characteristics of the TiN/Fe assembly are intensively studied. It is found that the adhesion strength is enhanced as one thin titanium layer is introduced in between. The orientation of the TiN layer is also modified by the Ti interlayer.Cr-N films, which exhibits better thermal and mechanical properties than TiN coating, is characterized. Phase identification indicates that films with lower nitrogen contents exhibit Cr and Cr2N phases, while the CrN phase is observed in films with nitrogen contents higher than 58 at.%. The microhardness of the deposited film ranges from 1500 to 1800HK at a load of 3gf with a Knoop indenter. For the oxidation test, Cr2O3 are formed on top of films. The phase analysis of the oxidized samples indicates that the pre-existed Cr and Cr2N phases in the deposited films are transformed to CrN due to the incorporation of nitrogen released by the oxidation layers. It is observed that the oxidation resistance of films with CrN phase is better than that with Cr or Cr2N phases.The addition of Al in TiN to form (Ti,Al)N ternary solid solution has been an attractive subject due to the enhancement of anti-oxidation and mechanical properties in comparison with TiN. It is found that the residual compressive stress in the as-deposited TiN turned into tensile as 39mol% of AlN is introduced to form Ti0.61Al0.39N. The brightness of the (Ti,Al)N, after oxidation experiment, is almost identical with that of the as-deposited coating, while that of TiN is much lower than those of (Ti,Al)N. Furthermore , the introduction of the Al atoms promotes the oxidation resistance of the (Ti,Al)N coating as compared to the pure TiN.
2.2.Transition Metal Nitride and Electroless Nickel Complex Coating
In the TiN/EN/MS system, electroless deposit is incorporated as an interlayer between TiN coating and mild steel substrate. The employment of EN results in an increase in hardness, the adhesion strength and corrosion resistance. The resultant surface hardness as high as 2120HK5 close to that of the bulk TiN can achieved in the complex coating system. Generally speaking, one single TiN layer can not protect the carbon steel substrate in the salt spray test since the general corrosion attack is found without the introduction of the EN interlayer. Although localized corrosion may be present for long period test, it is argued that this problem can be solved by introduction of an appropriate control in the fabrication of an uniform electroless nickel interlayer. Strengthening of the adhesion of the coating integrity is also evidenced.The electroless nickel layer crystallizes with the precipitation Ni3P phase during RF sputtering, and thus a coating assembly of CrN/Ni-Ni3P/MS is formed. The as-deposited electroless Ni-P coating raises the surface hardness of the steel substrate to more than three times. Surface hardness of the CrN coating modified by electroless Ni-P interlayer exhibits a hardness higher than 2000HK under a load of 15 gf. The usual substrate effect is nearly eliminated with the complex coating feature, and significant enhancement of surface hardness in the coating assembly is achieved. The corrosion tests indicate that the Ni-Ni3P/MS assembly exhibits a more positive Ecorr value (i.e. less electronegative) than CrN/MS and the corresponding potential curve is shifted toward the low-current side, indicating a better anti-corrosion performance. Through comparison in the Ecorr values and the polarization curves, it is demonstrated that CrN/Ni-Ni3P/MS exhibits significantly higher corrosion resistance than both the N-Ni3P/MS and CrN/MS coating assemblies.
2.3.Sputtered Nickel-Phosphorous-Based Ternary Coating
Electroless nickel is widely used as a hard coating for many industrial applications due to its high hardness, uniform thickness, corrosion and wear resistance. For advanced industrial applications, it is essential to promote the crystallization temperature of Ni-P deposits. In this study, Cu is introduced in Ni-P to improve the thermal stability. An alternative coating technique by r.f. magnetron sputtering is applied to deposit Ni-Cu-P and Ni-P-W coatings to solve the problem of composition control in the conventional chemical solution method. The Ni-Cu-P coatings were deposited by r.f. magnetron sputtering on 420 tool steel substrates with Cu+Ni-P compound targets. A novel design of the compound targets with Cu and Ni-P by consideration of the surface ratio of constituents exhibits a controllable composition in the deposited film. The compositions in the Ni-Cu-P coating can be modified by the original Ni-P deposits of the compound target along with the Cu area ratio in the compound target. The as-sputtered Ni-P-Cu deposits reveal the amorphous structure. The hardness of as-deposited Ni-P-Cu coatings decreases with increasing Cu contents. After annealing, the structure of the amorphous Ni-P-Cu deposits will transform directly into the Ni-Cu alloy and the Ni3P phase. The introduction of Cu in the ternary Ni-P-Cu deposits increases the crystallization temperature as compared with the binary Ni-P film. It is demonstrated that the thermal stability of the Ni-P-Cu films is enhanced with increasing Cu content in the sputtered deposits.The ternary Ni-P-W alloy coating was fabricated by the RF magnetron sputtering technique with dual targets of the electroless nickel alloy and the tungsten metal. The atomic ratio of the phosphorous to nickel in the coatings remained unchanged, regardless of the amount of the tungsten co-deposited. Transition in microstructure in terms of the tungsten contents in the as-deposited alloy deposit was discussed using X-ray diffraction analysis. Results in microhardness tests showed that the surface hardness could be engineered by the controlling the composition and microstructure in the Ni-P-W coating. A relatively high microhardness around 1900 HK was observed for the ternary coating with high tungsten contents of 65 wt.%. The thermal stability could be enhanced by addition tungsten into the deposit as compared to the binary Ni-P sputtered coating. After heat treatment, the sputtered ternary Ni-P-W coating was hardened by the Ni3P precipitation and solution hardening of tungsten in nickel. The introduction of the tungsten in the Ni-P coating by co-sputtering retarded the Ni3P precipitation and retained the strengthening effect to a higher temperature of 530 ℃.
2.4. Fabrication of Ir-Base Noble Metal Hard Coating
Ir-base noble metal hard coatings were deposited onto tungsten carbide (WC) substrate by r.f. magnetron sputtering process. The films were prepared by multi-target sputtering with iridium, rhenium and chromium as the sources. Argon and nitrogen were inlet into chamber to be the plasma and reactive gases, respectively. EPMA was employed to determine the composition of the hard coating. The relationship between the content of each component in the noble metal alloy films and the control of the input powers of sputtering guns was evaluated. The coating microstructure and morphology were also studied by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. It was shown that the surface roughness (Ra) values of uncoated substrate and Ir-Re-CrN protective thin film were in the same order, around 2~6nm, and both the surface morphologies were very uniform. Furthermore, there was no appreciable variation in the Ra value of the Ir-Re-CrN with the increase of chromium and nitrogen content. The film hardness was measured by Knoop microhardness test and that the multi-element coatings exhibited the superior hardness than the single noble metal hard films.
2.5.Surface Modification of Fe-Mn-Al Alloys by Chromization Process
FeMnAl alloy surface was modified by pack chromization process（Al2O3:Ferrochrominum powder: NH4Cl=67.5：30：2.5 in wt%）at 1000℃ for 1 to 16 hours. The (Cr,Fe)2N1-x and Cr23C6 phases are found on the chromized surface, while the internal part of chromized layer is Cr7C3 phase. The corrosion resistances of chromized alloy in 25℃, 3.5 wt% NaCl aqueous solution and in high temperature molten salt are greatly improved. In general, the corrosion resistance of Fe-Mn-Al alloy is enhanced by chromization process.The microhardness of the specimen chromized at 1000℃ for 4 hours reaches HK 2729 ± 148 under 10 gf load. The elastic modulus of chromized specimen measured by ICT is 473.9 ± 56.5Gpa. The chromized specimen possesses sufficient adhesion strength evaluated by scratch and ICT tests. The adhesion strength quality equals to HF1 or even better.Owing to very high toughness of Fe-Mn-Al alloy, it is suitable to produce heavy loading components, such as ship propeller and aircraft landing gear. It is promising for Fe-Mn-Al alloy to be applied in aerospace, automobile and ship industries. Since the corrosion resistance and mechanical properties of Fe-Mn-Al alloy are improved by pack chromization process, it is expected that the application field of chromized Fe-Mn-Al alloy can be extended to precision and mold industries.
2.6. Surface Modification of Aluminum Alloy by Electroless Nickel Plating
The purpose of this study is to derive electroless nickel plating on aluminum and its related alloys. The processes industries often use are anodic process and electrolytic method etc., but properties gained from both processes are limited in the small ranges of applications. However, electroless nickel plating layer has advantages of corrosion resistance, high hardness, wear resistance, lubricity, weldability and soderability, uniformity of the thickness and non-magnetic. Thus the process of electroless nickel plating is a better choice to meet the demands of materials properties. The application field is as follows:1. Increased in corrosion resistance of conventionally used umbrella can be achieved by anodic method, yet the hardness and strength fail to have any improvements. Electroless plating on umbrella ribs should have several benefits.2. The surface of aluminum heat sink needs one thin film for the heat protection. It can be easily plated with electroless plating for the heat protection, while it would be difficult to obtain good coverage on the internal surfaces electrolytically.
2.7. Surface Modification of the Bio-Grade Stents Materials
The microstructure and the electrochemical behavior of the passive film formed on bio-grade 316LVM stainless steel wires were investigated in this study. For as-received wires, oxide inclusions on the surface composed of alumina and calcium oxide showed harmful influence on bio-grade wires. Pickling in the improved solution with a mixture of HF/HNO3 and then in citrate acid could remove oxide inclusions effectively. The results of ESCA showed that the chromium oxide and hydroxide were the major oxides in the passive film, and several atomic layers of the hydrating water were absorbed on the surface. The deconvoluted peaks ratio of chromium-hydroxide to chromium-oxide at different sputtering time exhibited an exponential decay. As the wires were thermally oxidized, the ESCA spectra of thermally oxidized wires showed nearly no hydroxide in the passive film, while both chromium and iron oxide existed in the passive film. AES results indicated that the thickness of the passive films varies with the different passivated processes.