Sharpening of ceramic knife-edges and production of plasma for cutting tools nitriding with the use of a broad fast molecule beam

A source of gas molecules with energy ranging from 0.5 up to 4 keV has been used for production of non-self-sustained glow discharge plasma. Cutting tools immersed in nitrogen plasma are heated by the fast molecules but they are not sputtered by ions from the plasma. During nitriding of a tool the fast molecules sputter its cutting edges with the same rate as the rest of the tool surface. It results in a decrease of the edge radius. For sharpening of ceramic knife-edges were used fast neutral argon atoms, which sputter materials more intensively than nitrogen molecules.


Introduction
Ion-plasma processing is widely used in industry for strengthening the surface layer of various products [1].Hard wear-resistant coating can substantially increase the useful life of any product.This is due to the coating hardness of about 25 GPa [2], which exceeds the bulk material hardness by many times.Necessary for synthesis of titanium nitride and other hard coatings metal vapor is produced by means of vacuum arc evaporation [3] and magnetron sputtering [4].To prevent from droplets in the coatings the vacuum arc plasma can be filtered [5].Density of magnetron discharge plasma near the product surface and the sputtering rate are substantially enhanced when a pulsed dc magnetron [6] is used.Metal vapor can be also produced due to sputtering a target at the bottom of a hollow cathode [7].
It was shown in Reference [8] that under certain physical conditions superhard coatings with microhardness of 40-80 GPa and ultrahard coatings with microhardness of 80-105 GPa can be synthesized.Survey of those conditions is given in Reference [9].Usually the coating thickness amounts to a few microns.When its hardness is by an order of magnitude higher than hardness of the bulk material, the thin coating can be broken by external loads as a thin ice on the water surface or an egg shell.Performance and useful life of the coated product can be substantially improved when the thin coating is deposited a Corresponding author: a.metel@stankin.ruon a strengthened surface layer of the product, the layer being by an order of magnitude thicker than the coating.
Strengthening of the surface can be carried out, for instance, by means of nitriding.Nitrided surface layer exhibits a high loadbearing capacity and fatigue strength, thus preventing elastic and plastic deformations of the product as well as brittle rupture of synthesized coating [10].Combined processing of substrates made of highspeed steel in vacuum arc discharge plasma, which included nitriding and subsequent deposition of titanium nitride coatings, has been successfully carried out in Reference [11].Drawbacks of the arc method are soft metal droplets in the hard coatings, non-practical consumption of the target material and nonuniformity of the discharge plasma.To eliminate the droplets and the plasma nonuniformity it was proposed in References [12,13] to produce the nitrogen plasma in hollow cathode glow discharge [14,15] using the working vacuum chamber as the hollow cathode.
When a product immersed in any plasma is heated by ions accelerated from the plasma by negative voltage applied to the product, the ion current density on its surface depends on the product shape.It can be 10 to 100 times higher on its sharp edges than in the cavities.During 1hour-long pre-nitriding of cutting tools the thickness of surface layer sputtered from the tool cutting edge by the ions can exceed the mean thickness of the sputtered tool material by 10 to 100 times and amount to 20-30 μm.When initial radius of the cutting edge is equal to 15 μm, nitriding can result in an increase of the edge radius up to 45 μm, thus making the tool dull.

R e t r a c t e d
To prevent from the tool blunting, it is needed to ensure homogeneity of the tool surface sputtering.This paper is dedicated to production of a uniform nitrogen plasma and heating of the tools immersed in the plasma by a broad beam of fast nitrogen molecules [16].

Experimental setup
Figure 1 presents schematic of 500-mm-diameter vacuum chamber (1) equipped with a beam source of fast neutral molecules, which comprises 400-mm-diameter and 120-mm-deep titanium hollow cathode (2), 40-mmdiameter disc anode (3) and 300-mm-diameter emissive grid (4) made of 2-mm-thick titanium sheet.Distance between the centers of 4.6-mm-diameter orifices of the grid amounts to 5 mm and the grid transparency amounts to η = 0.8.Distance between grid (4) and a set of plates (5), which cover the opposite wall of chamber (1) preventing it from sputtering by fast molecules, amounts to 700 mm.A flat probe (not shown in Fig. 1) is introduced into chamber (1) to measure parameters of nitrogen plasma (6), the probe being parallel to the chamber axis and movable along and across the axis.Through the chamber bottom a rotating table (7) is introduced for placing products (8) to be processed by fast neutral molecule beam in plasma (6).
Ions (9) are accelerated between plasma emitter (10) filling cathode (2) and plasma (6) potential of the latter being by 2-3 V higher than the ground potential of chamber (1).For this reason energy of accelerated ions corresponds to measured with kilovoltmeter (11) voltage between anode (3) and chamber (1).At the gas pressure p = 0.2 -1 Pa and distance 0.1 m from grid (4) most of accelerated ions (9) turn into fast neutral molecules (12) due to charge exchange collisions [17] with gas molecules (13).Current of slow ions (14) in the circuit of chamber (1) through feedback resistor (15) with 3-kΩ resistance induces negative bias voltage 100-200 V of the grid thus preventing penetration of electrons from plasma (6) through grid (4) into positive space charge sheath (16) between grid (4) and plasma emitter (10).
Neglecting the secondary electron emission from grid (4) we may assume that beam current is equal to the sum of currents in chamber (1) and grid (4) circuits measured with ammeter (17) after being multiplied by the grid transparency η = 0.8.The energy of ions ( 9) is regulated using high-voltage power supply (18), the beam current is regulated using power supply (19) between anode (3) and cathode (2).Ion current in the circuit of cathode (2) is measured with ammeter ( 20) and cathode fall of potential is controlled with voltmeter (21).Using feedthrough (22) a 2-mm-thick flat electrode (23) is immersed in plasma 6 its surface area amounting to 0.03 m 2 .Electrode is connected to positive pole of power supply (24), its bias voltage can be regulated from zero up to 500 V and measured with voltmeter (25), the current in its circuit being measured with ammeter (26).

Experimental results
Figure 2 presents dependences of ion current density j i on the probe surface at the beam axis on the probe distance x from grid (4) at nitrogen pressure p = 0.2 Pa, equivalent beam current I b = 0.8 A and energy of molecules E b ranging from 1 to 4 keV.Hatched curves present j i (x) at I b = 0.8 A, E b = 4 keV and gas pressures up to 1.5 Pa.As the plasma density n is proportional to the ion current density presented in Figure 2 distributions show that n reaches its maximum at distance x = 0.05−0.1 m from the grid surface and monotonically diminishes with further increase in the distance.At the molecule energy of 1 keV axial distribution of the plasma density n is highly nonuniform and exhibits a fourfold decrease in n when x is increasing from zero to 0.5 m.After the molecule energy rises to 4 keV the plasma nonuniformity does not exceed ±30%.At the same energy of fast molecules the plasma density exhibits a twofold increase when the nitrogen pressure p rises from 0.2 Pa to 1.5 Pa.
The plasma density can be substantially increased and its homogeneity improved using a non-self-sustained glow discharge with electrostatic confinement of electrons [18] between chamber (1) and electrode (23) immersed in plasma (6) filling the chamber.Figure 4  E i = 15.8 eV of nitrogen [20].At one and the same U value velocity dI/dU is rising when the beam equivalent current I b and/or its energy E b are growing (Fig. 4).At U > 150 V the current I in all cases exceeds the doubled value of I o .It means that at U > 150 V electrons produce in the chamber more ions than fast neutral molecules.
Dash-dot curve in Figure 4 presents current-voltage characteristic of self-sustained glow discharge between anode (23) and chamber (1) playing the role of hollow cathode, when the beam source is switched off.It shows that only at the voltage U exceeding 300 V discharge supported by the beam does not expire after the beam source is switched off.Presented in Figure 5 radial distributions in the discharge plasma of ion current density j i in the probe circuit demonstrate a substantial rise of plasma density with increasing voltage U of the non-self-sustained discharge and decrease of plasma nonuniformity down to ±8%, which is quite suitable for nitriding of products.
Ten cutting plates (8) (Fig. 1) made of high-speed steel were positioned in the center of chamber (1) on rotating table (7).They were heated by the beam of fast neutral nitrogen molecules with 4-keV energy and 0.4-A equivalent current up to 500 • C and soaked in plasma (6) of non-self-sustained discharge at voltage U = 100 V and nitrogen pressure p = 0.5 Pa.Temperature of the plates was controlled with pyrometer IRCON and stabilized during the treatment by means of the beam current regulation.After the one-hour-long processing the initial microhardness 860 HV of the plates increased up to 1350 HV.Usual for plasma immersion ion nitriding increase of the mean radius of cutting edges was not observed.On the contrary the radius even decreased from 20 ± 3 μm down to 17 ± 3 μm.The experimentally found sharpening of  5), 250 ( 6) and 300 V (7).
the tool cutting edges suggested an idea to use processing with fast neutral molecules for sharpening of ceramic knife-edges (Fig. 6).
To prove the idea the knife blades made of ZrO 2ceramic were positioned on table (7) (Fig. 1).As no nitriding is needed for ceramics, the blades were processed without using the non-self-sustained discharge, and power supply (24) was switched off.As sharpening a knife means elimination of some material from its side surfaces near the cutting edge, the knifes were etched with a broad beam of fast argon atoms, which sputter materials more rapidly than fast nitrogen molecules [21].Radii of the cutting edges were measured using the optical 3D measuring system MicroCAD premium+ produced by GFMesstechnik GmbH (Germany).Presented in Figure 6 profiles of a ceramic knife-edge, their width w and height h being measured in μm, show that after a two-hour-long processing at argon pressure p = 0.2 Pa, beam equivalent current of 0.5-A and argon atoms energy of 4 keV the initial mean radius R = 6±1 μm of cutting edges diminished to 2 ± 1 μm.

Discussions
The data presented above prove that injection of a broad beam of fast neutral nitrogen molecules into a vacuum chamber results in production of quite nonuniform plasma.The plasma density grows with the beam equivalent current, energy and the nitrogen pressure.This fact reveals a leading role of fast neutral molecules in gas ionization.
It should be emphasized that all produced electrons and ions are coming to the chamber walls.To measure current I o of electrons produced in the chamber it is enough to increase the plasma potential at least up to 10-15 V (Fig. 3) using an electrode immersed in the plasma.Part of those electrons is produced as a result of the gas ionization and the rest are produced due to kinetic electron emission from the chamber walls bombarded by fast neutral molecules.Current of electrons produced due to ionization and current of emitted electrons are both proportional to the beam power.But the fist of them is growing with nitrogen pressure and the second is independent of the pressure.Hence, the ratio of the first current to the second is growing with gas pressure.At an adequate pressure the current of ions produced by fast molecules [19] may exceed two times and more the beam equivalent current.
Nonuniformity of the beam-produced plasma is a serious disadvantage in view of its application in technology.Fortunately it can be readily improved using non-selfsustained glow discharge with electrostatic confinement of electrons [18].At the discharge voltage U < 20 V electrons emitted due to chamber bombardment by fast neutral molecules practically do not ionize the gas.When U exceeds 20 V they are accelerated in the sheath (27) up to energy eU exceeding nitrogen ionization threshold and before reaching the discharge anode (23) pass inside the chamber a way, which is 50 times longer than the mean pass of oscillating inside the chamber electrons between their rejections in the sheath (27).Hence, every electron traveling to the anode has enough time to visit all the chamber corners to ionize the gas and spatial distribution of ionization expectancy in the chamber is quite homogeneous.
At the discharge voltage U exceeding 150 V fast electrons many times return back to the cathode sheath (27) and produce therein new electrons capable of ionizing the gas.Multiplication of fast electrons in the cathode sheath [14] is the basic process of electrostatic trap effect in glow discharge previously known as hollow cathode effect.It results in ionization intensity growth, electrons produce more ions than fast molecules, plasma density rises by an order of magnitude and its nonuniformity falls down to ±8% (Fig. 5).
Figure 6 can explain why the radius of cutting edge does not increase after nitriding in beam-produced plasma.When steel knife (1) is immersed in homogeneous plasma (2) and negatively biased, it is heated by ions (3) extracted from plasma (2) and accelerated in sheath (4) of positive space charge.At a bias voltage of 100-1000 V the sheath width exceeds 1 mm.For this reason the radius of cutting edge ∼10 μm is hundred times lower than the radius of plasma surface surrounding the knife-edge and current density of ions sputtering the edge is hundred times higher than on the rest surface of the knife.Intensive sputtering results in a substantial increase of the edge radius.
When a similar knife is immersed in the same plasma and isolated from the chamber, it is bombarded by plasma ions with energy of 10-20 eV corresponding to its floating potential.This energy is lower than the sputtering threshold and knife (6) can be sputtered only by homogeneous broad beam of fast neutral molecules (7), injected into plasma (2).When the blade is rotating in plasma (2) 703-page 5 the cutting edge is being sputtered with the same rate as other parts of the knife surface.It results in decrease of the cutting edge radius.Fast neutral argon atoms sputter with the same homogeneity both metals and dielectrics.This allows sharpening of knifes made of any materials including ZrO 2 -ceramics.

Conclusions
Active nitrogen plasma produced inside a working vacuum chamber due to injection of a broad fast neutral molecule beam is mainly produced due to the gas ionization by the fast neutral molecules [19].Non-self-sustained glow discharge with electrostatic confinement of electrons allows a multifold increase of the plasma density and a decrease of its nonuniformity down to ∼10%.
In contrast to conventional ion nitriding of cutting tools immersed in plasma at low nitrogen pressure, which usually results in an increase of the cutting edge radius, the above new method of tools strengthening in plasma produced by a broad beam of fast neutral molecules results in sharpening of the tools instead of blunting.
A broad beam of fast neutral argon atoms allows sharpening of knifes made of any materials including dielectric ceramics.

Fig. 6 .
Fig. 6.Profiles of a ceramic knife-edge before a processing by argon atoms with energy 4 keV and beam current 0.5 A (a) as well as after the processing (b).
R e t r a c t e dA.S. Metel: Mechanics & Industry 17, 703 (2016)