ICMCTF2012 Session B2-2: CVD Coatings and Technologies

Although the Chemical Vapor Deposition (CVD) technique is 160 years old there are still opportunities and challenges. The tool manufacturing industries have generated a growing demand for novel material applicable for large scale production.

Due to advances in CVD technology the transfer of new coating materials from lab scale to production size is made possible. These advances allow us to question results generated in the last decades of CVD. They will give rise for better understanding of the CVD processes and consequentially it will give new chances for novel coatings.

Because of its large application potential the main focus of this work will be on hard materials. A review of state of the art coatings and post treatments in tool manufacturing industries will be given. But what will be the next challenges for CVD?

Since many years diamond coatings are a well established technique to enhance the lifetime of cemented carbide tools in the machining of extremely abrasive materials like graphite, carbon fibre reinforced plastics (CFRP) and some aluminum alloys. The main advantage of crystalline diamond films in machining applications is their outstanding hardness and durability, but these properties also come along with an enormous brittleness of diamond coatings. So for further improvement of the coating and tool lifetime, we have to understand first the different failure mechanisms, which may lead to damages and delaminations of the diamond coating. Typical damages in diamond coatings during machining are the formation of small cracks in the film, which may expand along the grain boundaries and cause film delamination in large areas according to the brittleness of diamond.

The CemeCon AG developed Multilayer CVD diamond coatings to combine the unique durability of diamond coatings on cemented carbide tools in machining performances with an enhanced tolerance to small cracks and damages in the diamond film without catastrophic failure. These multilayer diamond coatings are alternating layers of tough micro-crystalline and smooth nano-crystalline films. The achieved difference in crystal size between these two types enables a homogeneous dissipation of the initial crack energy by continuous change of the crack growth direction as shown in the figure. The implementation of these multilayer coatings in the machining of the most challenging materials like CFRP led to a further tool performance improvement compared to completely micro- or nano-crystalline diamond films.

CemeCon will present the theory of crack growth in these multilayer coatings and some examples of the diamond film performance in industrial applications.

The boron-doped diamond films were synthesized using mode-conversion type microwave plasma CVD apparatus. The Si substrate was scratched by diamond powder and then cleaned ultrasonically in acetone solution. Reaction gases were used CH4 (15 SCCM) and H2 (100 SCCM). Vapor of B(OCH3)3, the boron source, was carried by H2 carrier gas into the vacuum chamber with its flow rate of 1 to 6 SCCM. Pressure was 20.0kPa and microwave power was 1.0 kW. Reaction time was fixed to 3 h. The surface and cross sectional morphologies of deposits were observed by SEM. Qualities of the deposits were estimated by Raman spectroscopy. Electrical resistivities were measured by the four-point probe method.

As a result of the SEM observation, the grain sizes of deposits were 1 to 3 μm. The maximum thickness of 6 μm was obtained for carrier gas flow rate ; 3 SCCM. From the Raman spectra of the deposits, the broad peak at about 500, 1230 cm＾-1 and the weak peak at 1333 cm＾ -1 were observed for each samples. These peaks due to including high concentration of boron in the films. In addition, the intensities of the peaks at 1333 cm＾ -1 were decreased with increasing of the carrier gas flow rate. As a result of the electrical resistivity measurements by the four-point probe method, the electrical resistivities of boron-doped diamond films decreased with increasing of carrier gas flow rate. The minimum electrical resistivity of 2.5×10＾ -1Ω・ cm was obtained for carrier gas flow rate ; 6 SCCM.

As a conclusion, the boron-doped diamond films were fabricated with each carrier gas flow rates. In the Raman spectra of the films, the peaks caused high boron inclusion were observed.

A novel design for neutron detectors based on the isotope ¹⁰B instead of ³He has been suggested by the European Spallation Source (ESS), to overcome the very limited availability of ³He. In the detector design, very large area aluminum blades are coated with a thin film containing high amounts of ¹⁰B. ¹⁰B₄C was chosen as the thin film material instead of pure ¹⁰B, since it is easier to handle in a deposition process and due to its high resistance towards oxidation and wear. Here we demonstrate the synthesis of thin, amorphous, boron-carbon films at low temperature (400-600 °C), by thermally activated CVD using the organoborane triethylboron, B(C₂H₅)₃, (TEB) as single precursor. Since the neutron detectors will be based on aluminum, there is an upper temperature limit of approximately 600 °C, which limits a number of possible CVD-processes and also the aluminum substrate prevents the use of BCl₃ as boron precursor. Deposition by TEB is done on both single crystalline Si (100) substrates and aluminum plates; the deposition rate at 600 °C was close to 1 µm/h in argon and 0.35 µm/h in hydrogen. The film density, as measured by X-ray reflectivity (XRR), for films deposited at 600 °C in hydrogen was 2.42 g/cm³ (97 % of bulk B₄C) and 2.14 g/cm³ (86 % of bulk) for films deposited at 600 °C in argon. The atomic content of the deposited films were analyzed by Time of Flight Elastic Recoil Detection Analysis (ToF-ERDA), films with B/C-ratio of 4.6 and 3.6 were deposited at 600 °C in hydrogen and argon respectively, the hydrogen content in the films was 3-4 at%, regardless of deposition ambient. Both the film composition and film density was found to vary significantly with deposition temperature and deposition ambient. Based on our results, a deposition mechanism for boron-carbon films from TEB, where the TEB molecule is decomposed to BH₃ and hydrocarbons, is suggested.

When ammonia is added in acetylene to form carbon films using thermal chemical vapor deposition, effects of different ammonia/acetylene ratios on the deposition rate and microstructures of carbon films are investigated. The deposition temperature, working pressure, and deposition time of the thermal CVD process were set to 1113 K, 8 kPa, and 25 min, respectively. The total mass flow rate of acetylene and ammonia were kept at 40 cm³/min, and five kinds of carbon films were prepared with the ammonia/acetylene ratio of 0, 4/36, 8/32, 12/28, 16/24, and 20/20. Experimental results indicate that the deposition rate of carbon films decreases as the ammonia/acetylene ratio increases. The mean crystallite size and ordered degree of carbon films increase with increasing the ammonia/acetylene ratio. Moreover, when the ammonia/acetylene ratio increases, the carbon films have more sp³ carbon atoms and shift to diamond -like. Few nitrogen and hydrogen atoms are incorporated into carbon films. The deposition rate of carbon films is proportional to the partial pressure of acetylene with a power of about eighth order, and thus, the pyrolysis of acetylene with added ammonia is controlled by the adsorption process of sixteen-carbon species on the substrate. The results of thermal CVD carbon deposition using acetylene and ammonia are compared with those using acetylene and nitrogen.

Approximately 30 to 40 years ago the group of Stan Vepřek of the University of Zurich described that thin films of polycrystalline silicon could be prepared utilizing a reversible chemical reaction. In their paper they described how pieces of silicon were exposed to a low-pressure high-density hydrogen plasma promoting the formation of volatile silicon hydride, this was then transported within the reactor to a hot substrate which caused the decomposition of the hydride and the deposition of the silicon film.

Hydrogen plasmas have also been used for chemical etching of various materials and, for example, in the case of the deposition of diamond films the preferential etching of sp² bonded carbon is considered to be of fundamental importance.

In this paper we describe the etching of a metal (Mo or Ni) by hydrogen which was plasma-activated in a RF hollow cathode where the water-cooled cylindrical cathode was lined with the appropriate metal. The metal hydride vapour generate in the plasma was directed to quartz substrates which were maintained temperatures in excess of 300 ^oC. The metal hydride was thermal decomposed and a thin film of the metal was deposited. A special substrate heater was constructed such that four quartz substrates could be simultaneously exposed to the metal hydride vapour, but with each substrate at a different temperature; each approximately 20^oC less than the neighbouring one. In this way, depositions under identical conditions could be carried out at the same time but at four different temperatures. We report the deposition rate as a function of the substrate temperature and the RF plasma power applied to the hollow cathode.