FIELD OF THE INVENTION
The present invention relates to a method of depositing erosion, corrosion and cavitation resistant duplex coating for depositing on hydro turbine components.
BACKGROUND OF THE INVENTION
Erosion, cavitation, abrasion are all wear mechanisms which lead to premature failure of the components in many industrial applications. Silt erosion consists of sliding as well as impact wear that typically presents in hydro turbine application, causes severe erosion and sometimes results in catastrophic failures, when combined with cavitation erosion phenomenon. In addition to that corrosion also plays additional role in accelerating erosion in few hydro sites. Many processes has been developed in house such as carbide based coating by HVOF, laser hardening and plasma nitriding/ nitro carburizing and futuristic technologies such as thin films by PVD process, out of which HVOF coatings are being successful on commercial scale. However, continual development on improvement of the erosion, corrosion and cavitation resistance of hydro turbine components is needed to increase their life further.

Processes having hard coating/soft substrate system has good resistance of sliding wear but poor resistance towards impact wear, whereas processes having moderate but gradual hardness has good resistance to impact wear but moderate resistance towards sliding wear. Failure of hard coating/soft substrate system under many erosion conditions is caused by either de-bonding of the coating from the substrate or the fracture of the coating itself. The coating properties as well as the load bearing capacity of a coating/substrate system are extremely important aspects which should be considered when analyzing the fracture behavior of these systems. Duplex coatings consisting of plasma nitriding of steel substrate and subsequent deposition of hard coatings by Physical Vapour Deposition has advantage of having hard coating over moderate but gradual hard layer of plasma nitriding. Duplex coating reportedly improve tribological properties as well as load support capability of steel substrate, however their erosion silt erosion and cavitation) and corrosion behavior has not been studied yet. The present work aims to develop the duplex process to have resistance toward both sliding and impact wear mechanism. The development can be useful for the application areas having sliding as well as impact wear mechanism such as silt erosion of hydro turbine components, heavy duty gear, cutting tools etc.
According to Indian Patent No.264355 there is provided a method of producing silt erosion and corrosion resistant coatings using liquid fuel based high velocity oxy fuel (HVOF) spraying system. The silt and corrosion resistant coating has been obtained by controlling the spraying parameter and automatic manipulation of the components as well as HVOF gun.

Indian Patent No. 246802 describes a method of polyurethane coating over HVOF coating for combating erosion of hydro turbine components. The method comprising the steps of applying a high velocity oxy fuel (HVOF) coating; then an intermediate metallic coating of Ni-AI using wire are spray process which is applied on the said HVOF coated surface to improve the surface roughness of the said coated surface; then an organic based primer is applied on the surface to get better adhesion followed by application of polyurethane coating layer which is produced by mixing the resin and hardener with an appropriate ratio.
US 20090297720 disclosed an erosion and corrosion resistant coating comprising a metallic binder, a plurality of hard particles, and a plurality of sacrificial particles by thermal spray processes. It also disclosed a method of improving erosion and corrosion resistance of a metal component comprising disposing on a surface of the metal component the foregoing erosion and corrosion resistant coating comprising, and a metal component comprising a metal surface and the foregoing erosion and corrosion resistant coating comprising a first surface and a second surface opposite the first surface, wherein the first surface is disposed on the metal component surface.
US 20050112399 describes materials and processes for providing erosion resistance to hydroelectric turbine components. These Erosion resistant coating compositions include hard particles in a metal matrix such as nickel-based, cobalt-based and iron-based matrices applied by a plating process for complex geometry or hard to access component surfaces or by thermal spray processes for line of sight applications.

US 20050112411 teaches an erosion resistant coating process and material for improvement in line-of-sight applications. The erosion resistant coating composition includes nanostructured grains of tungsten carbide (WC) and/or submicron sized grains of WC embedded into a cobalt chromium (CoCr) binder matrix. A high velocity air fuel thermal spray process (HVAF) is used to create thick coatings in excess of about 500 microns with high percentages of primary carbide for longer life better erosion resistant coatings. These materials and processes are especially suited for hydroelectric turbine components.
US Patent 5,030,06, in order to improve the resistance to erosion due to cavitation, as well as erosion caused by mechanical action of the soil and sand particles provides a water-turbine in which a water contactable portion of the water turbine, in particular a portion of each moving blade which tends to be eroded, is formed of a stainless steel containing 0.07 to 0.2 wt% of C, not more than 2 wt% of Si, 7 to 15 wt% of Mn, 1 to 7% of Ni, 10 to 25% of Cr, 0.1 to 3 Wt% of W, and the balance substantially Fe and inevitably accompanying impurities.
US Patent 4,793,871 teaches a method of improving surface wear qualities of metal components by gas nitriding or nitrocarburisisng includes the preliminary step of heating the component to the nitriding temperature in an atmosphere which is inert to the metal of the component.

A literature published by M/s Sulzer Metaplas, recommended a process of duplex treatment consisting of plasma nitriding combined with the respective PVD hard coating process to extend the life span of tools that are subjected to high wear. However, no background art has been found for development of duplex coating based on ternary metal nitrides by magnetron sputtering process over plasma nitrided surface of erosion, corrosion and cavitation resistance for hydro turbine components.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method of depositing an erosion, corrosion and cavitation resistant duplex coating on hydro turbine components.
Another object of the invention is to propose a method of depositing an erosion, corrosion and cavitation resistant duplex coating on hydro turbine components in which silt erosion, corrosion and cavitation resistance of steels used in hydro turbine components is enhanced.
A further object of the invention is to propose a method of depositing an erosion, corrosion and cavitation resistant duplex coating on hydro turbine components in which thin films of ternary metal nitrides are deposited by Pulsed Direct Current (DC) reactive magnetron sputtering on plasma nitrided surface of steel substrate used in hydro turbine components.

SUMMARY OF THE INVENTION
Accordingly, there is provided a method of depositing an erosion, corrosion and cavitation resistant duplex coating consisting of Plasma Nitriding followed by deposition of hard coating by pulsed DC magnetron sputtering PVD process on steel substrate used for hydro turbine component. Nano-composite coatings based on ternary metal nitrides such as Ti-Si-N and Zr-W-N over white layer free plasma nitrided surface exhibit improved erosion, corrosion properties over base material as well as individual processes of plasma nitriding and thin films.
Silt erosion resistance properties of duplex coated surface was improved by around 30-32 folds and cavitation resistance properties of duplex coated surface was improved by around 8-10 folds than uncoated base material as well as individual process of plasma nitriding and thin films. Corrosion resistance properties of duplex coated surface was found better than plasma nitrided surface.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Plasma nitrided layer clearly showing white layer free nitrided zone.
Figure 2: SEM micrograph of the TiSiN and Zr-W-N thin film.
Figure 3: Silt erosion test results.
Figure 4: Silt erosion damages on test samples.
Figure 5: Cavitation erosion test results
Figure 6: Comparison of corrosion behavior after 600 hrs.

DETAIL DESCRIPTION OF THE INVENTION
In establishing the duplex coating process, experiments have been carried out using plasma-ion- nitriding (PIN) facility followed by thin film deposition by DC magnetron sputtering system. Plasma Nitriding were carried out on samples to achieve white layer free nitrided layer of 200-250 microns. For each experiments samples have been ultrasonically cleaned using acetone to get rid of oil, dirt, grease etc. The samples have been loaded in the chamber keeping dummy samples having similar geometry for thermocouple attachment. Each experiment consists of following steps:
• Pre-condition: to check base vacuum (0.04mbar) and current leakage
• Pre-heating: upto 80°C temp for degassing.
• Pre-cleaning 1 cycle: sputtering with hydrogen at temp 350°C-400°C
• Pre-cleaning 2 cycle: sputtering with hydrogen and argon at temp 450°C-500°C.
• Nitriding for 24-32 hrs at optimized parameters consisting of temperature in the range of 530-550°C, process pressure of 1.5 mbar, N2:H2 ratio as 30:70.
• Cooling till room temperature.

All these parameters are programmed in system logic and it controls the parameters during each cycle. The plasma is being monitored during the process and any disturbance in plasma is controlled during the whole process especially during the nitriding cycle. After the nitriding cycle, samples of each batch are unloaded and preserved separately for next operation.
To achieve duplex coating, ternary metal nitrides such as Ti-Si-IM and Zr-W-N thin films were deposited on steel substrates by pulsed DC reactive magnetron sputtering. The sputtering targets were 99.99% pure Ti, Si, Zr and W disc (4" diameter and 5 mm thick) and plasma nitrided steel substrates were placed against these targets on the rotator cum heater. Initially the base pressure was generated upto 3 x 10"5 Torr. Before starting the deposition, the targets were pre-sputtered for 15 min with a shutter located in between the targets and the substrate. This shutter is also used to control the deposition time. Another pre sputtering has been carried out for substrate with negative biasing of 150 V for 30 minutes to get rid of any residual white layer formed by plasma nitriding process.
According to the invention, the sputtering is carried out with optimized process parameters using Nitrogen and Argon gases in a vacuum chamber at a temperature upto 300°C and process pressure in the range of 1.0-2.0 mbar. The deposition was carried out for 4-5 hrs to get thickness in the range of 10-15 urn. All the deposition was performed at a fixed substrate to target distance of 90 mm. The complete process cycle consists of following steps:

• Load the suitable targets and substrate sample/component in the chamber.
• Base Vacuum: to lower base pressure of the chamber upto 2 x 10"5 torr
• Heating: Heating the chamber upto process temperature and maintain the base pressure.
• Pre-sputtering of substrate with argon for 15-30 minutes to clean the substrate surface at pressure of 10mbar with negative bias of 150V.
• Pre-sputtering of Targets with argon for 10-15 minutes at pressure of lOmbar.
• Set the gas ratio as per process parameter and maintain the process pressure upto 5-20 mbar.
• Apply negative bias to substrate as per process parameter.
• Start co-sputtering of both the targets together maintaining process parameters upto specified time to deposit required ternary metal nitride.
• Cool to chamber till room temperature, release the vacuum and unload the substrate sample.

Characterization and testing
Each batch of samples has been first characterized for SEM, microstructure, case depth and thin film thickness study. Figure 1 shows micrograph of plasma nitrided layer, which clearly confirms the 200 micron thick white layer free plasma nitrided layer. Figure 2 show the SEM micrograph of TiSiN and ZrWN thin films. These micrographs show that TiSiN, ZrWN, thin film upoto 15 umuuu thickness was deposited successfully through magnetron sputtering process on white layer free plasma nitrided samples.
Silt erosion tests were carried out using a water jet pressure impingement erosion setup. It consists of convergent nozzle through which mineral sand of 180-250 urn size and hardness upto 1100 HV is added. The slurry passes through a 4 mm dia. X 40 mm long tungsten carbide throat, providing a jet velocity of 29.0 m/sec. Rectangular samples of size 60 x 50 x 6 mm are kept in front of a sand laden water jet at various angle ranging from 0° to 90° and sand laden water is passed in a controlled manner. In the present study the test samples were mounted at a fixed angle of 30° to the impinging jet. The flow parameters such as water flow rate, pressure (up stream of the tungsten nozzle), angle of impingement and sand flow rate are monitored. The pressure readings were monitored continuously for the complete experiment. At the end of the test, the samples were removed and ultrasonically cleaned and dried to remove

any slurry or debris. The silt erosion resistance of all the samples was determined by mass loss measurement. The samples were weighed before and after the erosion t4est to an accuracy of + 0.10 mg using a precision balance. Each test was repeated to determine the experimental reliability and error. The experimental error was within + 3%.
Testing Parameters
Water Flow : 20LPM
Inlet water Pressure : 10.Kg/cm2
Silt concentration : 7500 ppm
Erodent Size : 180-250 urn
Erodent quantity : 1.5Kg
Slurry velocity : 29m/sec
Nozzle to sample distance : 150mm
Angle of Impingement : 30°

Silt erosion behavior of the thin films has been evaluated and results are shown in Figures 3 and 4. The duplex coating has shown better erosion resistance properties compared to the individual coating process of Plasma Nitriding and PVD under laboratory conditions. Silt erosion resistance of duplex coating is found to be manifold better than base material as well as individual processes.
The cavitation erosion test has been carried out as per the ASTM G-32 standard. For the cavitation testing, button samples as per the standard were prepared from 20x20-x60mm block by wire cutting. The testing was carried out for 9 hrs. Weight loss has been measured after each 1 hr and results has been tabulated as cumulative weight loss vs time. These results were compared with 13-4 base samples. Cavitation erosion behavior of duplex coating has been evaluated and results are shown in Figure 5. Cavitation erosion resistance of duplex coating is found to be manifold (8-10 times) better than base material as well as individual processes.
Corrosion resistance studies are carried out using salt spray test for duplex coating. The Salt Spray Test equipment is designed as per the ASTM B 117 standard to assess the coating and materials to withstand corrosion due to atmospheric conditions. The samples were placed in the salt fog of synthetic sea

water at required temperature. The samples were constantly exposed to fog to assess its ability to withstand in the simulated conditions. Visual inspections were carried out at regular interval. Figure 6 shows the corrosion effect on duplex coated and plasma nitride sample after 600 hrs of exposure. The duplex coating shown improvement in corrosion properties than plasma nitrided sample.
The duplex coating has improvised the plasma nitriding surface by depositing thin film of nano composite hard coating for better erosion and corrosion resistance. The developed coating process can be used for hydro turbine components affected by silt and cavitation erosion as well as corrosion.

WE CLAIM :
1. A method of deposition of thin films of ternary metal nitrides by Pulsed Direct Current (DC) reactive magnetron sputtering on plasma nitrided surface to achieve duplex coating on martensitic 13Cr-4Ni and AISI 410, 16Cr-5Ni, austenitic 18Cr- 8Ni and manganese steel substrate used in forming hydro turbine components, wherein the ternary metal nitride such as Ti-Si-N and Zr-W-N thin film is deposited upto 15 microns on white layer free nitrided substrate and wherein the sputtering is carried out using nitrogen and argon gases in a vacuum chamber at a temperature upto 300°C with a target power in the range of 300-600 W and process pressure in the range of 1.0-2.0 mbar with the deposition time of 4-5 hrs to achieve film thickness of 10-15 microns.
2. The method as claimed in claim 1, wherein white layer free nitrided layer of more than 200 micron depth is achieved on the steel substrate by optimized plasma nitriding parameters.
3. The method as claimed in claim 3, wherein the negative biasing for substrate is maintained at 60-150V for removal of any white layer and for adhesion of thin film on plasma nitrided surface before start of deposition of thin film.

4. The method as claimed in claim 1, wherein silt erosion resistance properties of duplex coated surface is improved by around 30-32 folds than uncoated material.
5. The method as claimed in claim 1, wherein the cavitation resistance properties of duplex coated surface is improved by around 8-10 folds than uncoated base material.
6. The method as claimed in claim 1, wherein the corrosion resistance
properties of duplex coated surface is higher than plasma nitrided surface.