D. M. Cao

Characterization of microstructure and mechanical behavior of sputter deposited Ti-containing amorphous carbon coatings.

Abstract We report on the characterization of microstructure and mechanical properties of sputter deposited Ti-containing amorphous carbon (Ti-aC) coatings as a function of Ti composition. Ti-aC coatings have been deposited by unbalanced magnetron sputter deposition, in an industrial-scale four-target coating deposition system. The composition and microstructure of the Ti-aC coatings have been characterized in detail by combining the techniques of Rutherford backscattering spectrometry (RBS) and hydrogen elastic recoil detection (ERD), transmission electron microscopy (TEM), X-ray absorption near edge structure (XANES) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. At Ti compositions 8 at.%, XANES and EXAFS data indicate that the average Ti atomic bonding environment in Ti-aC coatings resembles that in cubic B1-TiC, consistent with TEM observation of precipitation of TiC nanocrystallites in the a-C matrix. Beyond the Ti dissolution limit, the Ti-aC coatings are nanocomposites with nanocrystalline TiC clusters embedded in an a-C matrix. A large scale, quasi one-dimensional composition modulation in the Ti-aC coatings was observed due to the particular coating deposition geometry. Elastic stiffness and hardness of the Ti-aC coatings were measured by instrumented nanoindentation and found to vary systematically as a function of Ti composition. Unlubricated friction coefficient of Ti-aC coatings against WC–Co balls was found to increase as the Ti composition increases. As Ti composition increases, the overall mechanical behavior of the Ti-aC coatings becomes more TiC-like.

Friction and wear characteristics of ceramic nanocomposite coatings: Titanium carbide/amorphous hydrocarbon

Friction and wear characteristics of titanium-containing amorphous hydrocarbon (Ti–C:H) coatings were measured during unlubricated sliding against WC–Co. These Ti–C:H coatings consist of nanocrystalline TiC clusters embedded in an amorphous hydrocarbon (a-C:H) matrix, i.e., they are TiC/a-C:H nanocomposites. The elastic modulus and hardness of the coatings exhibit smooth variations with increasing Ti composition. In contrast, a relatively abrupt transition occurs in the friction coefficient and wear rate of the coatings over a relatively narrow (20–30 at. %) Ti composition range. Our results reveal bimodal friction and wear behaviors for the TiC/a-C:H nanocomposites, a-C:H like at Ti compositions below 20%, and TiC like at Ti compositions above 30%. The two different wear mechanisms that operate as the volume fraction of nanocrystalline TiC clusters changes are discussed.

Ti atomic bonding environment in Ti-containing hydrocarbon coatings

We report characterization of the average Ti atomic bonding environment in Ti-containing hydrocarbon (Ti–C:H) coatings by x-ray absorption near edge structure (XANES) spectroscopy, extended x-ray absorption fine structure (EXAFS) spectroscopy, and high-resolution transmission electron microscopy (TEM). Ti–C:H coatings have been synthesized in a hybrid chemical vapor deposition/physical vapor deposition deposition system, which combines inductively coupled plasma and sputter deposition. Combining x-ray absorption spectroscopy with high resolution TEM imaging, we have determined that the dissolution limit of Ti atoms in an amorphous hydrocarbon (a-C:H) matrix is between 0.9 and 2.5 atomic percent. At Ti compositions >2.5 at. %, XANES and EXAFS data indicate that the average Ti atomic bonding environment in Ti–C:H resembles that in cubic B1–TiC, consistent with direct TEM observation of the precipitation of TiC nanocrystallites in an a-C:H matrix. Beyond the Ti dissolution limit, Ti–C:H coatings are in fact ...

Probing for mechanical and tribological anomalies in the TiC/amorphous hydrocarbon nanocomposite coating system

Abstract We have performed a detailed examination of the mechanical properties and the friction and wear characteristics of titanium-containing amorphous hydrocarbon (Ti–C:H) coatings as a function of the Ti composition from 0 to 45 at.%. Elastic modulus and hardness were measured with instrumented nanoindentation. Friction coefficient and coating wear rate during unlubricated sliding against WC–Co in a ball-on-disk configuration were also measured. Deposited by inductively coupled plasma (ICP)-assisted hybrid chemical vapor deposition/physical vapor deposition (CVD/PVD), these Ti–C:H coatings consist of nanocrystalline TiC clusters embedded in an a-C:H matrix, and are thin film TiC/amorphous hydrocarbon (a-C:H) nanocomposite materials. As the Ti composition increases, the elastic modulus and hardness of the coatings exhibit smooth variations. In contrast, relatively abrupt transitions occur in the friction coefficient and the wear rate of the coatings over a relatively narrow (20–30 at.%) Ti composition range. Our results indicate bimodal friction and wear behaviors for the TiC/a-C:H nanocomposites, a-C:H-like at Ti compositions below 20%, and TiC-like at Ti compositions above 30%. Our results indicate that two different coating wear mechanisms operate as the volume fraction of nanocrystalline TiC clusters changes. Over the entire range of Ti compositions examined, no anomalous changes in elastic modulus or hardness of the Ti–C:H coatings were observed.

Amorphous hydrocarbon based thin films for high-aspect-ratio MEMS applications

Abstract Methods to engineer the surfaces of micro-electro-mechanical systems (MEMS) are just beginning to be explored. Coating deposition is one such method. In cases where microdevices involve moving parts in contact, such as gear sets, combustion chambers, and mechanical seals, it may be necessary to impart the complex, highly non-planar surfaces with desired mechanical properties and tribological characteristics. In other cases, such as micro-heat exchangers or micro-catalytic converters, the chemical properties of the microdevice surfaces are important. Amorphous hydrocarbon (a-C:H) and metal-containing amorphous hydrocarbon (Me-C:H) thin films possess moderately high hardness, chemical inertness, low coefficient of friction and low wear rate, and are potentially useful in a wide range of microdevice applications. We report on the mechanical properties and tribological characteristics of TiC:H films as a function of the Ti composition. We further demonstrate the conformal deposition of TiC:H thin films over electrodeposited Ni high-aspect-ratio micro-scale structures (HARMs) fabricated by the deep X-ray lithography based microfabrication technique LIGA (Lithographie, Galvanoformung, Abformung). Conformal deposition of nanostructured ceramic thin films over HARMs offers possibilities for improving the tribological characteristics of HARMs based microdevices with parts in relative motion, and for using nanostructured ceramic thin films as structural materials for microdevices. As an example, we demonstrate the fabrication of freestanding, high-aspect-ratio TiC:H microtubes based on conformal deposition over electrodeposited Ni HARMs.

Conformal deposition of Ti-C:H coatings over high-aspect-ratio micro-scale structures and tribological characteristics

Results on conformal deposition of Ti-containing hydrocarbon (Ti-C:H) coatings over Ni high-aspect-ratio micro-scale structures (HARMs) fabricated by the lithography, electroplating and molding (LIGA) technique are reported. Using a high-density plasma assisted hybrid chemical vapor deposition/physical vapor deposition technique, it is demonstrated that conformal Ti-C:H deposition can be achieved over HARMs with ∼1000 μm in height and ∼220 μm in mutual spacing. Surface roughness and tribological characteristics of Ti-C:H coatings during unlubricated sliding contact with 52100 steel balls are studied with atomic force microscopy, micro-Raman scattering, and optical microscopy on macro-scale samples. It is shown that unlubricated sliding between Ti-C:H and 52100 steel is significantly influenced by the composition of the Ti-C:H coating.

Micromolding of Pb and Zn with Surface Engineered LiGA Mold Inserts

Molding of Pb and Zn metal plates with LiGA (Lithographie, Galvanoformung, Abformung) fabricated Ni micro-scale mold inserts was carried out with as fabricated Ni inserts and Ni inserts conformally coated with Ti-containing hydrocarbon (Ti-C:H) coatings. The molding performance was characterized using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS), in terms of the generated features on the metal plates as well as the inserts condition after molding. The present results demonstrate that, in cases where significant metal/insert chemical interactions exist, surface modification of the mold insert is necessary to obtain satisfactory performance. Furthermore, conformal deposition of suitable engineered ceramic coatings onto Ni micro-scale mold inserts is effective for high temperature micromolding of reactive metals.