a glow discharge apparatus suitable for preparing the abrasive material is shown in fig1 generally as 10 . the glow discharge apparatus 10 includes a vacuum chamber 12 which can be a glass bell jar . in the vacuum chamber 12 are two electrodes 14 and 18 which can be a screen , coil or plate of a material that is a good electrical conductor and does not readily sputter , such as aluminum . the electrodes 14 and 18 are connected to a power supply 16 , which may be dc or ac , to produce a voltage potential . behind the electrodes 14 and 18 are magnets 30 and 32 . these magnets 30 and 32 are typically electrically insulated from the electrodes 14 and 18 . an outlet 20 from the vacuum chamber 12 allows for evacuation of the system and is connected to a pumping station , not shown . a first inlet 22 and a second inlet 24 are connected to gas bleed systems , not shown , for adding the reactants employed to prepare the abrasive material . in carrying out the process , a substrate 26 to be coated , which overlies a ferromagnetic plate 28 , is placed between the electrodes 14 and 18 , typically maintained about 5 to 15 centimeters apart . the vacuum chamber 12 is then evacuated through the outlet 20 to a pressure of about 0 . 5 to 1 × 10 - 5 torr . a gas such as n 2 o is added through a first inlet 22 to its desired partial pressure . sih 4 is added through a second inlet 24 until the desired partial pressure ratio of sih 4 to n 2 o is obtained . in order to begin deposition of an abrasive coating on the substrate 26 , a glow discharge is initiated between the electrodes 14 and 18 by energizing the power source 16 . for deposition the current should be in the range of about 200 to 900 milliamps , preferably about 400 to 700 milliamps . any convenient frequency may be employed . the potential between electrodes 14 and 18 is generally about 1 , 000 volts . the magnets 30 and 32 , along with the energized electrodes 14 and 18 create , an electromagnetic field which keeps the glow in the vicinity of the electrodes 14 and 18 for a more efficient reaction of the precursors . in the past , great care had been taken to avoid using any materials in hardware or fixtures that would disturb this electromagnetic field . surprisingly however , it has been found that the silicon oxide deposition rate within the electrode area increases by placing the ferromagnetic plate 28 behind the lapping disc substrate 26 during deposition . fig2 is a lateral view showing a stationary test disc used to measure the thickness of material deposited inside the electrode area ( dotted lines indicate electrode area ). the invention will be further illustrated by the following example , but it is to be understood that the invention is not meant to be limited to the details described therein . a 30 . 5 centimeter diameter poly ( vinyl chloride ) test disc coated with a metal bi - layer comprising a copper layer about 50 angstroms thick and a 200 angstrom thick layer of inconel 600 ( 79 . 0 percent by weight of nickel , 13 . 8 percent by weight of chromium and 8 . 33 percent by weight of iron ) was placed in a 46 centimeter by 76 centimeter bell jar as shown in fig1 . the disc substrate was mounted with a cold - rolled steel 35 cm diameter backing plate between two 15 centimeter by 15 centimeter aluminum electrodes . these electrodes covered a strip approximately 12 centimeters wide on the disc . the bell jar was then evacuated to a pressure of 1 × 10 - 5 torr . n 2 o was added to a partial pressure of 32 micrometers of mercury using a flow rate as estimated by nitrogen flow of 23 . 6 standard cubic centimeters per minute ( sccm ). sih 4 was then added to a partial pressure of 4 . 5 micrometers at a flow rate of 4 . 3 sccm . to create a glow between the electrodes , 500 milliamps of current were supplied to the electrodes with a potential of about 1 , 000 volts at 10 kilohertz for 10 minutes . the rates of deposition for various positions within the electrode area shown in fig2 were measured . the results are averaged in table 1 . a second 30 . 5 centimeter pvc test disc was coated as above except that no ferromagnetic backing plate was used . the rates of deposition were measured and the averaged results follow in table 1 . table 1______________________________________ average rage of deposition ( within electrode area ) ______________________________________example 1 150 å / minutecontrol 1 106 å / minute______________________________________