Deposition Technique and Parameters

A SPLD620-FLR PVD coating machine (manufactured by Plasmionique Inc) was used for coating deposition purposes employed a reactive RF magnetron sputtering method. The substrate holder was equipped by a stationary heating etching station for heating and etching purposes during deposition. The configuration of the SPLD620-FLR coating system is illustrated in Figure 3-2. The following sections describe the configuration of the SPLD620-FLR coating system and the experimental set-up.

28 _{The degree of purity of technical gases is given in the form x.y. The number x stands for the number of "9",} which are separated after the second decimal place. The number y gives the last one decimal place. So, a purity of 5.0 corresponds to a purity of 99.9990 vol.%.

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Figure 3-2: The configuration of the SPLD620-FLR coating system. The system employed different match boxes to match the power between power generators and the two RF-

powered magnetron sources.

In the coating system, the deposition can only take place in the RF mode (frequency 13.56 MHz). Hence, the deposition of the binary Cr-O and ternary Cr-Zr-O coatings was performed in RF mode. In RF mode, a matching network is needed, consisting of two capacitors and one coil, so that the output impedance of the generators is matched to the input impedance of the magnetron sources and therefore the transfer of power between the generators and the magnetrons is maximized. To generate the alternating voltages, three RF generators (made by Seren, Model R601 and R301), were used. Two RF generators 600 W (13.56 MHz) were available for two magnetron sources and one RF generator 300 W (13.56 MHz) was dedicated to substrate for biasing purposes.

The heating etching station is a substrate heater which to its heating disk a substrate bias can be applied and is stationary in the coating chamber. The samples are positioned on top of it in predefined position to achieve uniform coatings and thus cannot be moved during the individual process steps. The heating element is made of SiC and has a diameter of 10 cm. This SiC element can be heated by resistive heating up to 900°C. The heater conductor is connected to a RF supply which can apply up to 100 W power to create a desired bias during deposition.

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Temperature measurement is done using a Type K thermocouple enclosed in an Inconel sheath and positioned close to the substrate. It should be noted that the actual substrate temperature may differ slightly from the measured temperature since the thermocouple is not in direct contact with the substrate.

Two separate sets of experiments were designed for deposition of binary Cr-O and ternary Cr-Zr- O coatings. For all the depositions the chamber was evacuated to a base pressure of better than 10 -4 _{Pa. The target–substrate distance was kept to 50 mm. To remove any chemical contaminants on} the substrate surface, the substrates were etched at a DC equivalent voltage of -150 V, corresponding to a RF power of about 50 W, for 20 min in a pure Ar plasma at 0.3 Pa pressure. At the same time, the targets were sputtered at 300 W for 20 min to remove impurities at the target surface. The shutters in front of targets were closed during the target cleaning period to prevent any deposition on substrates. Immediately after the substrate bias is turned off and the target cleaning is finished, the substrate grounded, the shutters moved away, and pure Ar replaced by a mixture of Ar and O2 and the coating process begins. The following explains the experiments designed for each coating system:

a) Binary Cr-O coatings:

In this set of experiments, the effect of deposition parameters including deposition pressure, temperature, Cr-target voltage, and Ar/O2 ratio on the structure, phase composition, and mechanical properties of chromium oxide coatings was investigated. First, depositions were performed at room temperature (without external substrate heating) using a Cr target voltage of 260 V. The Ar/O2 ratio was 6, while the deposition pressure varied from 1 to 1.6 × 10−1 Pa. In the second series, the deposition temperature altered between room temperature and 400 °C, with deposition pressure constant at 1.6 × 10−1 Pa. In the third series, Cr target voltage was tuned from 180 V to 300 V, while deposition pressure and deposition temperatures were 1.6 × 10−1 Pa and room temperature, respectively. In the final series, the Ar/O2 ratio changed between 6 and 3, whereas deposition pressure, Cr target voltage, and temperature remained constant at 1.6 × 10−1 Pa, 260 V, and room temperature, respectively. Detailed deposition parameters are summarized in Table 3-2. This experimental design allows for the investigation of the individual effects of each deposition parameters on the structure and mechanical properties of chromium oxide coatings.

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Table 3-2: Detailed deposition parameters for chromium oxide coatings.

Parameter Cr Voltage (V) Ar Flow Rate (sccm) O2 Flow Rate (sccm) Temperature (°C) Pressure (Pa) Pressure Change 260 30 5 25 1 0.82 0.29 0.16 Temperature Change 260 30 5 25 0.16 150 300 400 Cr Voltage Change 300 30 5 25 0.16 260 220 180

Ar/O2 Ratio Change 260

30

5 25 0.16

25 20 15

b) Ternary Cr-Zr-O coatings:

In this ternary system, depositions were performed at the optimum parameters determined from the previous step (a) in which chromium oxide coatings with a hardness value close to bulk Cr2O3 (H~29 GPa) had been produced, i.e. at working pressure of 0.16 Pa, Cr-target voltage of 260 V, and a mixture of Ar and O2 with 6/1 ratio, respectively. The influence of the composition and substrate temperature, varied in 150°C steps from 25°C to 850°C, on structure, phase

composition, and mechanical properties of Cr-Zr-O coatings were evaluated. Table 3-3 shows a summary of the deposition conditions for the prepared oxide coatings in Cr-Zr-O system.

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Table 3-3: Deposition conditions for Cr-Zr-O coatings.

Cr Power (W) Zr Power (W) Ar flow rate (sccm) O2 flow rate (sccm) Temperature (o C) Pressure (Pa) Deposition time (h) 300 0 30 5 25-850 0.16 24 300 50 30 5 25-850 0.16 24 300 75 30 5 25-850 0.16 24 300 100 30 5 25-850 0.16 24 300 125 30 5 25-850 0.16 24