Cutting elements for incorporation in a drill bit are provided having a body having an end face interfacing with an ultra hard material cutting layer. A main depression having a nonplanar surface is formed on the substrate and extending to the edge of the substrate subjected to the highest impact loads during drilling. This edge is immediately below the edge of the cutting layer which makes direct contact with the earth formations during drilling. The main depression is formed by forming a plurality of secondary depressions or steps such that depth of the main depression decreases in a radial direction towards the edge of the cutting element. An ultra hard material layer is bonded to the end face of the cutting element body such that the cutting layer is the thickest over the main depression. Consequently, the cutting layer edge making contact with the earth formations during drilling has an increased thickness.

FIELD OF THE INVENTION
 This invention relates to cutting elements used in drag bit for drilling
 earth formations. Specifically this invention relates to cutting elements
 having a nonplanar interface including a non-uniform portion between their
 substrate and their cutting layer.
 BACKGROUND OF THE INVENTION
 A typical cutting element is shown in FIG. 1. The cutting element typically
 has cylindrical cemented carbide substrate body 2 having an end face or
 upper surface 3. An ultra hard material layer 4, such as polycrystalline
 diamond or polycrystalline cubic boron nitride, is bonded on to the upper
 surface forming a cutting layer. The cutting layer can have a flat or a
 curved upper surface 5.
 The problem with many cutting elements is the development of cracking,
 spoiling, chipping and partial fracturing of the ultra hard material
 cutting layer at the layer's region subjected to the highest impact loads
 during drilling. This region is referred to herein as the "critical
 region". These problems are caused by the generation of peak (high
 magnitude) stresses imposed on the ultra hard material layer at the
 critical region during drilling. Because the cutting elements are
 typically inserted into a drag bit at a rake angle, the critical region
 includes a portion of the ultra hard material layer near to and including
 a portion of the layer's circumferential edge 6.
 Another problem facing cutting elements is the delamination and/or the
 exfoliation of the ultra hard material layer from the substrate of the
 cutting element resulting in the failure of the cutter. Delamination
 and/or exfoliation become more prominent as the thickness of the diamond
 layer increases.
 SUMMARY OF THE INVENTION
 The present invention provides for cutting elements or inserts which are
 mounted in a bit body and which have an increased thickness of the ultra
 hard material cutting layer at their critical region, i.e., the region of
 the cutting element subjected to the highest impact loads during drilling.
 This region is generally defined beginning at the edge of the cutting
 element which contacts the earth formations during drilling and can span
 up to 50% of the cross-sectional area of the cutting element. Preferably,
 the critical region extends to an area between 45.degree. and 70.degree.
 on either side of the point of contact of the cutting element with the
 earth formation and inward to an area near the central axis of the cutting
 element.
 A main depression is formed on the body (i.e., the substrate) end face
 (i.e., the upper surface) of the cutting element. The main depression is
 defined by multiple secondary depressions defining a main depression
 surface having a depth which increases in a generally stepwise fashion in
 an outward radial direction along a critical diameter and which decreases
 arcuately on either side of the critical diameter. The critical diameter
 is the diameter that intersects the point of contact between the edge of
 the cutting element and the earth formation. An ultra hard material layer
 is bonded to the end surface of the substrate and has either a curved or
 flat upper surface such that an increased thickness of the ultra hard
 material layer is formed over the critical region with the maximum ultra
 hard material thickness occurring at or proximate the edge portion or edge
 point of the cutting element making contact with the earth formations
 during drilling.
 Moreover, a circumferential groove is formed on the outer surface of the
 body of the cutting element and spans an arc that is approximately the
 same as the are spanned by the critical region of the cutting element. The
 groove is preferably symmetric about the critical diameter of the cutting
 element. An ultra hard material is packed into the groove forming a
 secondary cutting surface for improving the cutting efficiency of the
 cutting element as well as delaying the erosion of the cutting element
 during drilling.

DETAILED DESCRIPTION OF THE INVENTION
 A present invention cutting element 1 (i.e., insert) has a body (i.e., a
 substrate) 10 having a curved upper surface 12 (FIGS. 9, 2A and 2B). The
 body is typically cylindrical. A circumferential edge 14 is formed at the
 interface of the curved upper surface 12 and the cylindrical outer surface
 16 of the body. An ultra hard material layer such a polycrystalline
 diamond or cubic boron nitride layer 80 is formed on top of the upper
 surface (FIGS. 7A and 7B). The cutting elements 1 are mounted in a bit
 body 7 along a rake angle 8 (FIGS. 8 and 9). Consequently, the cutting
 layer 58 of each cutting element makes contact with the earth formation
 ideally at an edge point 9 referred to herein as the "cutting layer
 critical point". Similarly, the point on the body circumferential edge 14
 axially below the cutting layer cricital point is referred to herein as
 the "body critical point" 19. To resist cracking, spoiling, chipping and
 partial fracturing, the present invention places an increased thickness of
 ultra hard material at a region 18 of cutting element which is subjected
 to the highest impact loads during drilling. This region, referred to
 herein as the "critical region" includes the body critical point 19. The
 critical region spans less than 50% of the circular cross-sectional area
 of the cutting element. Typically, however, the critical region spans an
 arc of about 45.degree. to 70.degree. on either side of the body critical
 point 19 and extend radially from the circumference of the cutting element
 inward to a location at or near the cutting element central axis and in
 many instance may extend over the central axis. A typical critical region
 90 is shown bounded by dashed line 92 as shown in FIG. 2C. With the
 cutting elements of the present invention, the thickness of the ultra hard
 material cutting layer is increased where it is needed and minimized in
 other places so as to minimize the risk of cutting layer delamination
 and/or exfoliation.
 To increase the thickness of the ultra hard material layer at the critical
 region, a main depression 20 is formed within the critical regions at the
 curved upper surface 12 (i.e., the end face) of the body that interfaces
 with the cutting ultra hard material cutting layer. The main depression is
 formed by forming a series of smaller adjacent depressions. For
 illustrative purposes, these smaller depressions are referred to herein as
 "secondary depressions". These adjacent secondary depressions can
 sometimes define steps. The main depression typically spans an arc less
 than 180.degree. along the substrate upper surface. Preferably, however,
 the depression spans an arc 22 between 90.degree. and 135.degree. on the
 body upper surface (FIG. 2B). Because the critical region and the main
 depression spans only a portion of the insert body upper surface, and
 because the body upper surface is generally curved the body upper surface
 is unplanar and non-axisymmetric. Hence the cutting elements of the
 present invention are referred to as "unplanar non-axisymmetric inserts".
 In one embodiment, the main depression is formed by a series of relatively
 shallow groove-like secondary depressions 21, 23, and 25 (FIGS. 2A-2D).
 The main depression 20 is ovoidal in shape. The secondary depressions are
 symmetric about a diameter 29. For descriptive purposes, the diameter is
 referred to herein as the "critical diameter." The critical diameter is
 the diameter that intersects the critical point 19. Each depression has a
 maximum actual depth 32--as measured between the highest and lowest point
 of the depression--occurring along the critical diameter. Each of the
 secondary depressions arcuately span the main depression defining
 scalloped edges 24 and 26 on the main depression as shown in FIG. 2C.
 Ridges 11 are formed between adjacent secondary depressions. The ridges 11
 preferably have rounded apexes 13 to reduce stress concentrations. In this
 embodiment, the ridges 11 are arcuate curving toward the center of the
 insert. The curvature or radius of curvature of each ridge decreases for
 each subsequent radially outward ridge.
 The depth 28 of each subsequent secondary depression--as measured from a
 reference point 30 on the substrate upper surface--is increased in a
 direction toward the body critical point 19 along the critical diameter 29
 as shown in FIG. 2B. In other words, the height of the lowest surface of
 each subsequent secondary depression as measured from a reference plane 94
 perpendicularly intersecting the body decreases radially outward along the
 critical diameter 29. In this regard, each secondary depression defines a
 step.
 In another embodiment, the main depression 33 is generally triangular in
 shape (FIG. 3A). With this embodiment, the secondary depressions 31 are
 arranged in a pyramidal fashion defining a surface resembling the negative
 of a turtle shell. Three secondary depressions 132, 232, 332 are arranged
 along the circumferential edge of the insert body upper surface defining a
 first row of secondary depressions. A second row comprising two secondary
 depressions 134, 234 is formed adjacent to the first row of secondary
 depressions. Each of the second row secondary depressions is adjacent to
 two secondary depressions from the first row. A secondary depression 136
 defines a third row. The third row secondary depression 136 is adjacent to
 both of the second row depressions 134, 234. In a preferred embodiment,
 the surface defined by the secondary depressions is symmetric about a
 critical diameter 35 of the insert body. Thus, in the preferred
 embodiment, the end depressions 132, 332 of the first row are mirror
 images of each other. Similarly the two second row depressions 134, 234
 are also mirror images of each other, whereas the third row depression 136
 and the middle depression 232 of the first row are both symmetric about
 the critical diameter. The maximum depth of the defined surface occurs
 along the critical diameter 35 with the first row middle depression 232
 having a depth as measured from a point 32 on the upper surface of the
 body that is greater than the depth of the third row depression 136 as
 measured from the same point. The intersections between consecutive
 secondary depressions form ridges 37 which have preferably rounded apexes
 39 so as to reduce stress concentrations (FIG. 3B).
 In another embodiment, the main depression 72 is formed by a series of
 generally radial secondary depressions 74 which preferably increase in
 depth--as measured from a reference point 79 on the substrate upper
 surface--in a direction toward the body circumferential edge 14 (FIGS. 4A
 and 4B). These depressions may be shaped and arranged to define a main
 depression surface that resembles the inner surface of a shell or that
 resembles a fan, as shown in FIG. 4A. Ridges 75 with preferably rounded
 apexes 77 are formed between adjacent secondary depressions. The main
 depression surface defined by the secondary depressions is symmetric about
 a critical diameter 73 of the insert body. Moreover, the maximum depth of
 the surface defined by the secondary depressions--as measured from a
 reference point 79 on the upper surface of the insert body--occurs at the
 body critical point 19. Furthermore, the maximum depth of each secondary
 depression decreases for depressions further away from the diameter. In
 addition, each radial secondary depression may itself consist of multiple
 steps or depressions, as for example steps 76 and 78 shown in FIG. 4A.
 In yet another embodiment, the secondary depressions forming the main
 depression 50 comprise steps and concave walls. The steps and walls formed
 are arranged in a shape resembling a terrace or an amphitheater 46 which
 is symmetric about a critical diameter 49 as shown in FIGS. 5A and 5B. In
 this embodiment, of a series of arcuate steps 51, 53, 55, and 57 define
 the central part of the main depression as shown in FIGS. 5A and 5B. These
 steps define arcuate depressions which curve toward the body critical
 point 19. The radius of curvature decreases for each subsequent radially
 outward arcuate step. The edges 80 of these steps and the edges 81 between
 steps are rounded to reduce stress concentration. A series of concave
 walls 52, 54, 56, 58 and 59 surround the arcuate depressions and extend to
 the periphery of the main depression 50. The maximum depth of each
 depression--as measured from a reference point 47 on the substrate upper
 surface--increases in a radially outward direction along the critical
 diameter 49. Moreover, the maximum depth of each depression occurs along
 the critical diameter 49.
 In a further embodiment, the secondary depressions forming the main
 depression 60 comprise arcuate concave walls 63 and a series of arcuate
 steps 62, 64, 66 and 68 (FIGS. 6A and 6B). These steps define arcuate
 depressions which curve toward the center of the cutting element body.
 Each step is relatively flat. The radius of curvature decreases for each
 subsequent radially inward step. The main depression 60 does not extend to
 the central axis 69 of the cutting element. The arcuate concave walls 63
 interconnect the subsequent steps and extend to the peripheral edges of
 the main depression 60. An arcuate concave wall 65 bounds the step 62
 closest to the insert central axis. An edge 67 of the concave wall 65
 curves around the central axis 69 of the substrate. The steps and walls
 are symmetric about a critical diameter 70. Moreover the depth of each
 subsequent step--as measured from a reference point on the upper surface
 of the body--increases for each subsequent radially outward step.
 With all of the aforementioned embodiments, the periphery of the main
 depression which is defined by the secondary depressions, steps or walls,
 is scalloped. Moreover, the ultra hard material layer 58 bonded to the
 upper surface of the substrate may have a flat upper surface 81 (FIG. 7A)
 or may have a convexly curved or dome-shaped upper surface 82 (FIG. 7B),
 while the layer lower surface which interfaces with the substrate upper
 surface is complementary to the substrate upper surface. As such, an ultra
 hard material cutting layer is formed on top of the substrate having
 increased thickness at the critical region and a maximum thickness at the
 critical point of the cutting layer that will make contact with the earth
 formations during drilling. The maximum thickness of the ultra hard
 material cutting layer is preferably in the range of 0.08 to 0.12 inch.
 The minimum thickness of the ultra hard material layer is preferably in
 the range of 0.06 to 0.08 inch.
 In all of the aforementioned embodiments, the multiple secondary
 depressions used to define the main depression provide for an non-planar
 interface between diamond cutting layer and the substrate in the critical
 region. This non-planar main depression provides for a larger bonding area
 between the ultra hard material and the body so as to reduce the stress
 levels at the interface which cause delamination. Consequently, a thicker
 ultra hard material cutting layer portion may be bonded at the critical
 region without increasing the risk of delamination of the cutting layer.
 Furthermore, with any of these embodiments, a circumferential groove 83 may
 be formed on the cylindrical outer surface of the substrate in a location
 below the depression (FIGS. 3A, 3B, 4A, 4B, 5A, 5B, 7A, and 7B).
 Preferably, the groove spans an arc equal or slightly greater than the arc
 22 span by the main depression on the upper surface of the substrate. In
 cross-section, preferably the groove has a horizontal upper side wall 84
 and a slanted lower side wall 85 with a round bottom 86 therebetween. The
 slanted lower side wall slants in the direction opening the groove. The
 circumferential groove is preferably symmetric about a plane through the
 critical diameter. Applicant has discovered that the geometry of this
 groove reduces the level of the stresses generated at and around the
 groove. Moreover, the slanted wall of the groove provides for a groove
 geometry that is easier to pack with ultra hard material, thereby making
 the manufacture of the cutting clement easier and less costly.
 Ultra hard material is bonded into the circumferential groove forming a
 secondary cutting surface 88. This secondary cutting surface serves two
 purposes. First it serves as an additional cutting surface, increasing the
 cutting efficiencies of the cutting element. Second, it delays the erosion
 and wear of the cutting element body that occurs when the cutting element
 body is allowed to make contact with the earth formation during drilling.
 All of the inserts of the present invention are mounted in a bit body 7 and
 are oriented such that the critical region of each insert is positioned to
 engage the earth's formation at the critical point 9 of the cutting layer
 which will make contact with the earth formation during drilling (FIG. 8).
 In this regard, the region of high impact loading during cutting will have
 the thickest section of ultra hard material. Moreover, by doing so, the
 secondary cutting surface will also be aligned to eventually contact the
 earth formation and increases cutting efficiency as well as delay the
 erosion and wear of the cutting element.